Systems and methods for automated detection in magnetic resonance images

ABSTRACT

Some aspects include a method of determining change in size of an abnormality in a brain of a patient positioned within a low-field magnetic resonance imaging (MRI) device. The method comprises, while the patient remains positioned within the low-field MRI device, acquiring first and second magnetic resonance (MR) image data of the patient&#39;s brain; providing the first and second MR image data as input to a trained statistical classifier to obtain corresponding first and second output; identifying, using the first output, at least one initial value of at least one feature indicative of a size of the abnormality; identifying, using the second output, at least one updated value of the at least one feature; determining the change in the size of the abnormality using the at least one initial value of the at least one feature and the at least one updated value of the at least one feature.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit under 35 U.S.C. § 119(e) of U.S.Provisional Application Ser. No. 62/425,569, titled “CHANGE DETECTIONMETHODS AND APPARATUS”, filed on Nov. 22, 2016, which is incorporated byreference herein in its entirety.

BACKGROUND

Magnetic resonance imaging (MRI) provides an important imaging modalityfor numerous applications and is widely utilized in clinical andresearch settings to produce images of the inside of the human body. MRIis based on detecting magnetic resonance (MR) signals, which areelectromagnetic waves emitted by atoms in response to state changesresulting from applied electromagnetic fields. For example, nuclearmagnetic resonance (NMR) techniques involve detecting MR signals emittedfrom the nuclei of excited atoms upon the re-alignment or relaxation ofthe nuclear spin of atoms in an object being imaged (e.g., atoms in thetissue of the human body). Detected MR signals may be processed toproduce images, which in the context of medical applications, allows forthe investigation of internal structures and/or biological processeswithin the body for diagnostic, therapeutic and/or research purposes.

MRI provides an attractive imaging modality for biological imaging dueto its ability to produce non-invasive images having relatively highresolution and contrast without the safety concerns of other modalities(e.g., without needing to expose the subject to ionizing radiation, suchas x-rays, or introducing radioactive material into the body).Additionally, MRI is particularly well suited to provide soft tissuecontrast, which can be exploited to image subject matter that otherimaging modalities are incapable of satisfactorily imaging. Moreover, MRtechniques are capable of capturing information about structures and/orbiological processes that other modalities are incapable of acquiring.However, there are a number of drawbacks to conventional MRI techniquesthat, for a given imaging application, may include the relatively highcost of the equipment, limited availability (e.g., difficulty andexpense in gaining access to clinical MRI scanners), the length of theimage acquisition process, etc.

The trend in clinical MRI has been to increase the field strength of MRIscanners to improve one or more of scan time, image resolution, andimage contrast, which in turn drives up costs of MRI imaging. The vastmajority of installed MRI scanners operate using at least at 1.5 or 3tesla (T), which refers to the field strength of the main magnetic fieldB0 of the scanner. A rough cost estimate for a clinical MRI scanner ison the order of one million dollars per tesla, which does not evenfactor in the substantial operation, service, and maintenance costsinvolved in operating such MRI scanners.

Additionally, conventional high-field MRI systems typically requirelarge superconducting magnets and associated electronics to generate astrong uniform static magnetic field (B0) in which a subject (e.g., apatient) is imaged. Superconducting magnets further require cryogenicequipment to keep the conductors in a superconducting state. The size ofsuch systems is considerable with a typical MRI installment includingmultiple rooms for the magnetic components, electronics, thermalmanagement system, and control console areas, including a speciallyshielded room to isolate the magnetic components of the MRI system. Thesize and expense of MRI systems generally limits their usage tofacilities, such as hospitals and academic research centers, which havesufficient space and resources to purchase and maintain them. The highcost and substantial space requirements of high-field MRI systemsresults in limited availability of MRI scanners. As such, there arefrequently clinical situations in which an MRI scan would be beneficial,but is impractical or impossible due to the above-described limitationsand as discussed in further detail below.

SUMMARY

Some embodiments are directed to a method of detecting change in degreeof midline shift in a brain of a patient positioned within a low-fieldmagnetic resonance imaging (MRI) device, the method comprising: whilethe patient remains positioned within the low-field MRI device:acquiring first magnetic resonance (MR) image data of the patient'sbrain; providing the first MR data as input to a trained statisticalclassifier to obtain corresponding first output; identifying, from thefirst output, at least one initial location of at least one landmarkassociated with at least one midline structure of the patient's brain;acquiring second MR image data of the patient's brain subsequent toacquiring the first MR image data; providing the second MR image data asinput to the trained statistical classifier to obtain correspondingsecond output; identifying, from the second output, at least one updatedlocation of the at least one landmark associated with the at least onemidline structure of the patient's brain; and determining a degree ofchange in the midline shift using the at least one initial location ofthe at least one landmark and the at least one updated location of theat least one landmark.

Some embodiments are directed to a low-field magnetic resonance imagingdevice configured to detect change in degree of midline shift in a brainof a patient positioned within a low-field magnetic resonance imaging(MRI) device, the low-field MRI device comprising: a plurality ofmagnetic components, including: a B0 magnet configured to produce, atleast in part, a B0 magnetic field; at least one gradient magnetconfigured to spatially encode magnetic resonance data; and at least oneradio frequency coil configured to stimulate a magnetic resonanceresponse and detect magnetic components configured to, when operated,acquire magnetic resonance image data; and at least one controllerconfigured to operate the plurality of magnet components to, while thepatient remains positioned within the low-field magnetic resonancedevice, acquire first magnetic resonance (MR) image data of thepatient's brain, and acquire second MR image data of the patient's brainsubsequent to acquiring the first MR image data, wherein the at leastone controller further configured to perform: providing the first andsecond MR data as input to a trained statistical classifier to obtaincorresponding first output and second output; identifying, from thefirst output, at least one initial location of at least one landmarkassociated with at least one midline structure of the patient's brain;identifying, from the second output, at least one updated location ofthe at least one landmark associated with the at least one midlinestructure of the patient's brain; and determining a degree of change inthe midline shift using the at least one initial location of the atleast one landmark and the at least one updated location of the at leastone landmark.

Some embodiments are directed to at least one non-transitorycomputer-readable storage medium storing processor-executableinstructions that, when executed by at least one computer hardwareprocessor, cause the at least one computer hardware processor to performa method of detecting change in degree of midline shift in a brain of apatient positioned within a low-field magnetic resonance imaging (MRI)device. The method comprises, while the patient remains positionedwithin the low-field MRI device, acquiring first magnetic resonance (MR)image data of the patient's brain; providing the first MR data as inputto a trained statistical classifier to obtain corresponding firstoutput; identifying, from the first output, at least one initiallocation of at least one landmark associated with at least one midlinestructure of the patient's brain; acquiring second MR image data of thepatient's brain subsequent to acquiring the first MR image data;providing the second MR image data as input to the trained statisticalclassifier to obtain corresponding second output; identifying, from thesecond output, at least one updated location of the at least onelandmark associated with the at least one midline structure of thepatient's brain; and determining a degree of change in the midline shiftusing the at least one initial location of the at least one landmark andthe at least one updated location of the at least one landmark.

Some embodiments are directed to a system comprising: at least onecomputer hardware processor; and at least one non-transitorycomputer-readable storage medium storing processor-executableinstructions that, when executed by the at least one computer hardwareprocessor, cause the at least one computer hardware processor to performa method of detecting change in degree of midline shift in a brain of apatient positioned within a low-field magnetic resonance imaging (MRI)device. The method comprises, while the patient remains positionedwithin the low-field MRI device, acquiring first magnetic resonance (MR)image data of the patient's brain; providing the first MR data as inputto a trained statistical classifier to obtain corresponding firstoutput; identifying, from the first output, at least one initiallocation of at least one landmark associated with at least one midlinestructure of the patient's brain; acquiring second MR image data of thepatient's brain subsequent to acquiring the first MR image data;providing the second MR image data as input to the trained statisticalclassifier to obtain corresponding second output; identifying, from thesecond output, at least one updated location of the at least onelandmark associated with the at least one midline structure of thepatient's brain; and determining a degree of change in the midline shiftusing the at least one initial location of the at least one landmark andthe at least one updated location of the at least one landmark.

Some embodiments are directed to a method of determining change in sizeof an abnormality in a brain of a patient positioned within a low-fieldmagnetic resonance imaging (MRI) device, the method comprising: whilethe patient remains positioned within the low-field MRI device:acquiring first magnetic resonance (MR) image data of the patient'sbrain; providing the first MR image data as input to a trainedstatistical classifier to obtain corresponding first output;identifying, using the first output, at least one initial value of atleast one feature indicative of a size of an abnormality in thepatient's brain; acquiring second MR image data of the patient's brainsubsequent to acquiring the first MR image data; providing the second MRimage data as input to the trained statistical classifier to obtaincorresponding second output; identifying, using the second output, atleast one updated value of the at least one feature indicative of thesize of the abnormality in the patient's brain; determining the changein the size of the abnormality using the at least one initial value ofthe at least one feature and the at least one updated value of the atleast one feature.

Some embodiments are directed to a low-field magnetic resonance imaging(MRI) device configured to determine change in size of an abnormality ina brain of a patient, the low-field MRI device comprising: a pluralityof magnetic components, including: a B0 magnet configured to produce, atleast in part, a B0 magnetic field; at least one gradient magnetconfigured to spatially encode magnetic resonance data; and at least oneradio frequency coil configured to stimulate a magnetic resonanceresponse and detect magnetic components configured to, when operated,acquire magnetic resonance image data; and at least one controllerconfigured to operate the plurality of magnet components to, while thepatient remains positioned within the low-field magnetic resonancedevice, acquire first magnetic resonance (MR) image data of thepatient's brain, and acquire second MR image data of the patient's brainsubsequent to acquiring the first MR image data, wherein the at leastone controller further configured to perform: providing the first andsecond MR image data as input to a trained statistical classifier toobtain corresponding first output and second output; identifying, usingthe first output, at least one initial value of at least one featureindicative of a size of an abnormality in the patient's brain; acquiringsecond MR image data for the portion of the patient's brain subsequentto acquiring the first MR image data; identifying, using the secondoutput, at least one updated value of the at least one featureindicative of the size of the abnormality in the patient's brain;determining the change in the size of the abnormality using the at leastone initial value of the at least one feature and the at least oneupdated value of the at least one feature.

Some embodiments are directed to at least one non-transitorycomputer-readable storage medium storing processor-executableinstructions that, when executed by at least one computer hardwareprocessor, cause the at least one computer hardware processor, toperform method of determining change in size of an abnormality in abrain of a patient positioned within a low-field magnetic resonanceimaging (MRI) device, the method comprising: while the patient remainspositioned within the low-field MRI device: acquiring first magneticresonance (MR) image data of the patient's brain; providing the first MRimage data as input to a trained statistical classifier to obtaincorresponding first output; identifying, using the first output, atleast one initial value of at least one feature indicative of a size ofan abnormality in the patient's brain; acquiring second MR image data ofthe patient's brain subsequent to acquiring the first MR image data;providing the second MR image data as input to the trained statisticalclassifier to obtain corresponding second output; identifying, using thesecond output, at least one updated value of the at least one featureindicative of the size of the abnormality in the patient's brain;determining the change in the size of the abnormality using the at leastone initial value of the at least one feature and the at least oneupdated value of the at least one feature.

Some embodiments are directed to a system, comprising: at least onecomputer hardware processor; at least one non-transitorycomputer-readable storage medium storing processor-executableinstructions that, when executed by the at least one computer hardwareprocessor, cause the at least one computer hardware processor, toperform method of determining change in size of an abnormality in abrain of a patient positioned within a low-field magnetic resonanceimaging (MRI) device. The method comprises, while the patient remainspositioned within the low-field MRI device, acquiring first magneticresonance (MR) image data of the patient's brain; providing the first MRimage data as input to a trained statistical classifier to obtaincorresponding first output; identifying, using the first output, atleast one initial value of at least one feature indicative of a size ofan abnormality in the patient's brain; acquiring second MR image data ofthe patient's brain subsequent to acquiring the first MR image data;providing the second MR image data as input to the trained statisticalclassifier to obtain corresponding second output; identifying, using thesecond output, at least one updated value of the at least one featureindicative of the size of the abnormality in the patient's brain; anddetermining the change in the size of the abnormality using the at leastone initial value of the at least one feature and the at least oneupdated value of the at least one feature.

Some embodiments are directed to a method of detecting change inbiological subject matter of a patient positioned within a low-fieldmagnetic resonance imaging (MRI) device, the method comprising: whilethe patient remains positioned within the low-field MRI device:acquiring first magnetic resonance image data of a portion of thepatient; acquiring second magnetic resonance image data of the portionof the patient subsequent to acquiring the first magnetic resonanceimage data; aligning the first magnetic resonance image data and thesecond magnetic resonance image data; and comparing the aligned firstmagnetic resonance image data and second magnetic resonance image datato detect at least one change in the biological subject matter of theportion of the patient.

Some embodiments are directed to a low-field magnetic resonance imagingdevice configured to detecting change in biological subject matter of apatient positioned with the low-field magnetic resonance imaging device,comprising: a plurality of magnetic components, including: a B0 magnetconfigured to produce, at least in part, a B0 magnetic field; at leastone gradient magnet configured to spatially encode magnetic resonancedata; and at least one radio frequency coil configured to stimulate amagnetic resonance response and detect magnetic components configuredto, when operated, acquire magnetic resonance image data; and at leastone controller configured to operate the plurality of magnet componentsto, while the patient remains positioned within the low-field magneticresonance device, acquire first magnetic resonance image data of aportion of the patient, and acquire second magnetic resonance image dataof the portion of the patient subsequent to acquiring the first magneticresonance image data, the at least one controller further configured toalign the first magnetic resonance image data and the second magneticresonance image data, and compare the aligned first magnetic resonanceimage data and second magnetic resonance image data to detect at leastone change in the biological subject matter of the portion of thepatient.

Some embodiments are directed to at least one non-transitorycomputer-readable storage medium storing processor-executableinstructions that, when executed by at least one computer hardwareprocessor, cause the at least one computer hardware processor to performa method of detecting change in biological subject matter of a patientpositioned within a low-field magnetic resonance imaging (MRI) device,the method comprising: while the patient remains positioned within thelow-field MRI device: acquiring first magnetic resonance image data of aportion of the patient; acquiring second magnetic resonance image dataof the portion of the patient subsequent to acquiring the first magneticresonance image data; aligning the first magnetic resonance image dataand the second magnetic resonance image data; and comparing the alignedfirst magnetic resonance image data and second magnetic resonance imagedata to detect at least one change in the biological subject matter ofthe portion of the patient.

Some embodiments are directed to a system, comprising: at least onecomputer hardware processor; and at least one non-transitorycomputer-readable storage medium storing processor-executableinstructions that, when executed by the at least one computer hardwareprocessor, cause the at least one computer hardware processor to performa method of detecting change in biological subject matter of a patientpositioned within a low-field magnetic resonance imaging (MRI) device,the method comprising: while the patient remains positioned within thelow-field MRI device: acquiring first magnetic resonance image data of aportion of the patient; acquiring second magnetic resonance image dataof the portion of the patient subsequent to acquiring the first magneticresonance image data; aligning the first magnetic resonance image dataand the second magnetic resonance image data; and comparing the alignedfirst magnetic resonance image data and second magnetic resonance imagedata to detect at least one change in the biological subject matter ofthe portion of the patient.

BRIEF DESCRIPTION OF THE DRAWINGS

Various aspects and embodiments of the disclosed technology will bedescribed with reference to the following figures. It should beappreciated that the figures are not necessarily drawn to scale.

FIG. 1 is a schematic illustration of a low-field MRI system, inaccordance with some embodiments of the technology described herein.

FIGS. 2A and 2B illustrate bi-planar magnet configurations for a B₀magnet, in accordance with some embodiments of the technology describedherein.

FIGS. 2C and 2D illustrate a bi-planar electromagnet configuration for aB₀ magnet, in accordance with some embodiments of the technologydescribed herein.

FIGS. 2E and 2F illustrate bi-planar permanent magnet configurations fora B₀ magnet, in accordance with some embodiments of the technologydescribed herein.

FIGS. 3A and 3B illustrate a transportable low-field MRI system suitablefor use with change detection techniques described herein, in accordancewith some embodiments of the technology described herein.

FIGS. 3C and 3D illustrate views of a portable MRI system, in accordancewith some embodiments of the technology described herein.

FIG. 3E illustrates a portable MRI system performing a scan of the head,in accordance with some embodiments of the technology described herein.

FIG. 3F illustrates a portable MRI system performing a scan of the knee,in accordance with some embodiments of the technology described herein.

FIG. 3G illustrates another example of a portable MRI system, inaccordance with some embodiments of the technology described herein.

FIG. 4 illustrates a method of performing change detection, inaccordance with some embodiments of the technology described herein.

FIG. 5 illustrates a method of modifying acquisition parameters based onchange detection information, in accordance with some embodiments of thetechnology described herein.

FIG. 6 illustrates a method of co-registering MR image data, inaccordance with some embodiments of the technology described herein.

FIG. 7A illustrates a midline shift measurement, in accordance with someembodiments of the technology described herein.

FIG. 7B illustrates another midline shift measurement, in accordancewith some embodiments of the technology described herein.

FIG. 8 illustrates a method for determining a degree of change in themidline shift of a patient, in accordance with some embodiments of thetechnology described herein.

FIGS. 9A-C illustrate a convolutional neural network architectures formaking midline shift measurements, in accordance with some embodimentsof the technology described herein.

FIG. 10 illustrates fully convolutional neural network architectures formaking midline shift measurements, in accordance with some embodimentsof the technology described herein.

FIGS. 11A-11F illustrate measurements that may be used to determine thesize of a hemorrhage of a patient, in accordance with some embodimentsof the technology described herein.

FIGS. 12A-C illustrate measurements that may be used to determine achange in the size of a hemorrhage of a patient, in accordance with someembodiments of the technology described herein.

FIG. 13 illustrates a method for determining a degree of change in thesize of an abnormality (e.g., hemorrhage) in the brain of a patient, inaccordance with some embodiments of the technology described herein.

FIG. 14 illustrates a fully convolutional neural network architecturefor making measurements that may be used to determine the size of anabnormality (e.g., hemorrhage) in a patient's brain, in accordance withsome embodiments of the technology described herein.

FIG. 15 illustrates a convolutional neural network architecture formaking measurements that may be used to determine the size of anabnormality (e.g., a hemorrhage) in a patient's brain, in accordancewith some embodiments of the technology described herein.

FIG. 16 is a diagram of an illustrative computer system on whichembodiments described herein may be implemented.

DETAILED DESCRIPTION

The MRI scanner market is overwhelmingly dominated by high-fieldsystems, and particularly for medical or clinical MRI applications. Asdiscussed above, the general trend in medical imaging has been toproduce MRI scanners with increasingly greater field strengths, with thevast majority of clinical MRI scanners operating at 1.5 T or 3 T, withhigher field strengths of 7 T and 9 T used in research settings. As usedherein, “high-field” refers generally to MRI systems presently in use ina clinical setting and, more particularly, to MRI systems operating witha main magnetic field (i.e., a B₀ field) at or above 1.5 T, thoughclinical systems operating between 0.5 T and 1.5 T are often alsocharacterized as “high-field.” Field strengths between approximately 0.2T and 0.5 T have been characterized as “mid-field” and, as fieldstrengths in the high-field regime have continued to increase, fieldstrengths in the range between 0.5 T and 1 T have also beencharacterized as mid-field. By contrast, “low-field” refers generally toMRI systems operating with a B₀ field of less than or equal toapproximately 0.2 T, though systems having a B₀ field of between 0.2 Tand approximately 0.3 T have sometimes been characterized as low-fieldas a consequence of increased field strengths at the high end of thehigh-field regime. Within the low-field regime, low-field MRI systemsoperating with a B₀ field of less than 0.1 T are referred to herein as“very low-field” and low-field MRI systems operating with a B₀ field ofless than 10 mT are referred to herein as “ultra-low field.”

As discussed above, conventional MRI systems require specializedfacilities. An electromagnetically shielded room is required for the MRIsystem to operate and the floor of the room must be structurallyreinforced. Additional rooms must be provided for the high-powerelectronics and the scan technician's control area. Secure access to thesite must also be provided. In addition, a dedicated three-phaseelectrical connection must be installed to provide the power for theelectronics that, in turn, are cooled by a chilled water supply.Additional HVAC capacity typically must also be provided. These siterequirements are not only costly, but significantly limit the locationswhere MRI systems can be deployed. Conventional clinical MRI scannersalso require substantial expertise to both operate and maintain. Thesehighly trained technicians and service engineers add large on-goingoperational costs to operating an MRI system. Conventional MRI, as aresult, is frequently cost prohibitive and is severely limited inaccessibility, preventing MRI from being a widely available diagnostictool capable of delivering a wide range of clinical imaging solutionswherever and whenever needed. Typically, patient must visit one of alimited number of facilities at a time and place scheduled in advance,preventing MRI from being used in numerous medical applications forwhich it is uniquely efficacious in assisting with diagnosis, surgery,patient monitoring and the like.

As discussed above, high-field MRI systems require specially adaptedfacilities to accommodate the size, weight, power consumption andshielding requirements of these systems. For example, a 1.5 T MRI systemtypically weighs between 4-10 tons and a 3 T MRI system typically weighsbetween 8-20 tons. In addition, high-field MRI systems generally requiresignificant amounts of heavy and expensive shielding. Many mid-fieldscanners are even heavier, weighing between 10-20 tons due, in part, tothe use of very large permanent magnets and/or yokes. Commerciallyavailable low-field MRI systems (e.g., operating with a B₀ magneticfield of 0.2 T) are also typically in the range of 10 tons or more duethe large of amounts of ferromagnetic material used to generate the B₀field, with additional tonnage in shielding. To accommodate this heavyequipment, rooms (which typically have a minimum size of 30-50 squaremeters) have to be built with reinforced flooring (e.g., concreteflooring), and must be specially shielded to prevent electromagneticradiation from interfering with operation of the MRI system. Thus,available clinical MRI systems are immobile and require the significantexpense of a large, dedicated space within a hospital or facility, andin addition to the considerable costs of preparing the space foroperation, require further additional on-going costs in expertise inoperating and maintaining the system.

In addition, currently available MRI systems typically consume largeamounts of power. For example, common 1.5 T and 3 T MRI systemstypically consume between 20-40 kW of power during operation, whileavailable 0.5 T and 0.2 T MRI systems commonly consume between 5-20 kW,each using dedicated and specialized power sources. Unless otherwisespecified, power consumption is referenced as average power consumedover an interval of interest. For example, the 20-40 kW referred toabove indicates the average power consumed by conventional MRI systemsduring the course of image acquisition, which may include relativelyshort periods of peak power consumption that significantly exceeds theaverage power consumption (e.g., when the gradient coils and/or RF coilsare pulsed over relatively short periods of the pulse sequence).Intervals of peak (or large) power consumption are typically addressedvia power storage elements (e.g., capacitors) of the MRI system itself.Thus, the average power consumption is the more relevant number as itgenerally determines the type of power connection needed to operate thedevice. As discussed above, available clinical MRI systems must havededicated power sources, typically requiring a dedicated three-phaseconnection to the grid to power the components of the MRI system.Additional electronics are then needed to convert the three-phase powerinto single-phase power utilized by the MRI system. The many physicalrequirements of deploying conventional clinical MRI systems creates asignificant problem of availability and severely restricts the clinicalapplications for which MRI can be utilized.

Accordingly, the many requirements of high-field MRI renderinstallations prohibitive in many situations, limiting their deploymentto large institutional hospitals or specialized facilities and generallyrestricting their use to tightly scheduled appointments, requiring thepatient to visit dedicated facilities at times scheduled in advance.Thus, the many restrictions on high field MRI prevent MRI from beingfully utilized as an imaging modality. Despite the drawbacks ofhigh-field MRI mentioned above, the appeal of the significant increasein SNR at higher fields continues to drive the industry to higher andhigher field strengths for use in clinical and medical MRI applications,further increasing the cost and complexity of MRI scanners, and furtherlimiting their availability and preventing their use as ageneral-purpose and/or generally-available imaging solution.

The inventors have developed techniques for producing improved quality,portable and/or lower-cost low-field MRI systems that can improve thewide-scale deployability of MRI technology in a variety of environmentsbeyond the large MRI installments at hospitals and research facilities.The inventors have appreciated that the accessibility and availabilityof such low-field MRI systems (e.g., due to the relatively low cost,transportability, etc.) enables imaging applications not available ornot practicable with other imaging modalities. For example, generallytransportable low-field MRI systems may be brought to a patient tofacilitate monitoring the patient over an extended period of time byacquiring a series of images and detecting changes occurring over theperiod of time. Such a monitoring procedure is not realistic withhigh-field MRI. In particular, as discussed above, high-field MRIinstallments are generally located in special facilities and requireadvanced scheduling at significant cost. Many patients (e.g., anunconscious Neural ICU patient) cannot be taken to an available facilityand, even if a high-field MRI installment can be made available, thecost of an extended MRI analysis over the course of multiple hours isgoing to be prohibitively expensive.

Furthermore, while CT scanners are generally more available andaccessible than high-field MRI systems, these systems still may not beavailable for relatively long monitoring applications to detect ormonitor changes that the patient is undergoing over an extended period.Moreover, an extended CT examination subjects the patient to asignificant dose of X-ray radiation, which may be unacceptable in many,if not most, circumstances. Finally, CT is limited in its ability todifferentiate soft tissue and may be incapable of detecting the type ofchanges that may be of interest to a physician. The inventors haverecognized that low-field MRI facilitates performing monitoring tasks incircumstances where current imaging modalities cannot do so.

The inventors have recognized that the transportability, accessibilityand availability of low-field MRI systems permits monitoringapplications that are not available using existing imaging modalities.For example, low-field MRI systems can be used to continuously and/orregularly image a portion of anatomy of interest to detect changesoccurring therein. For example, in the neuro-intensive care unit (NICU),patients are often under general anesthesia for a significant amount oftime while the patient is being assessed or during a procedure. Becauseof the need for a specialized facility, conventional clinical MRIsystems are not available for these and many other circumstances. Inaddition, physicians may only have limited access to a computedtomography (CT) device for a patient (e.g., once a day). Moreover, evenwhen such systems are available, it is inconvenient and sometimesimpossible to image patients that are, for example, unconscious orotherwise not able to be transported to the MRI facility. Thus,conventional MRI is not typically used as a monitoring tool.

The inventors have recognized that low-field MRI can be used to monitora patient by acquiring magnetic resonance (MR) image data over a periodof time and detecting changes that occur. For example, a transportablelow-field MRI system can be brought to a patient that can be positionedwithin the system while a sequence of images of the patient's brain isacquired. The acquired images can be aligned and differences betweenimages can be detected to monitor any changes taking place. Imageacquisition may be performed substantially continuously (e.g., with oneacquisition immediately performed after another), regularly (e.g., withprescribed pauses in between acquisitions), or periodically according toa given acquisition schedule. As a result, a physician may obtaintemporal information concerning physiology of interest. For example, thetechniques described herein may be used to monitor a patient's brain todetect change in the degree of midline shift in the brain. As anotherexample, the techniques described herein may be used to monitor apatient's brain to detect change in the size of an abnormality (e.g., ahemorrhage in the brain).

Accordingly, the inventors have developed low-field MRI techniques formonitoring a patient's brain for changes related to a brain injury,abnormality, etc. For example, the low-field MRI techniques describedherein may be used to determine whether there is a change in a degree ofmidline shift for a patient. Midline shift refers to an amount ofdisplacement of the brain's midline from its normal symmetric positiondue to trauma (e.g., stroke, hemorrhage, or other injury) and is animportant indicator for clinicians of the severity of the brain trauma.

In some embodiments, low-field MRI monitoring techniques may be combinedwith machine learning techniques to continuously monitor the amount ofmidline displacement in a patient (if any) and detect a change in thedegree of the midline shift over time. In such embodiments, low-fieldMRI monitoring allows for obtaining a sequence of images of a patient'sbrain and the machine learning techniques (e.g., deep learningtechniques such as convolutional neural networks) may be used todetermine, from the sequence of images, a corresponding sequence oflocations of the brain's midline and/or a corresponding sequence of themidline's displacements from its normal position. For example, in someembodiments, deep learning techniques may be used to identify locationsof the points where the falx cerebri is attached to the inner table ofthe patient's skull and a location of a measurement point in the septumpellucidum. These locations may in turn be used to obtain a midlineshift measurement.

It should be appreciated, however, that although in some embodiments themidline is detected by detecting locations of the attachment points ofthe falx cerebri, there are other ways of detecting the midline. Forexample, in some embodiments, the midline may be detected by segmentingthe left and right brain and the top and bottom part of the brain (asdefined by the measurement plane).

In some embodiments, midline shift monitoring involves, while thepatient remains positioned within a low-field MRI device: (1) acquiringfirst magnetic resonance (MR) image data for a portion of the patient'sbrain; (2) providing the first MR data as input to a trained statisticalclassifier (e.g., a convolutional neural network) to obtaincorresponding first output; (3) identifying, from the first output, atleast one initial location of at least one landmark associated with atleast one midline structure of the patient's brain; (4) acquiring secondMR image data for the portion of the patient's brain subsequent (e.g.,within one hour) to acquiring the first MR image data; (5) providing thesecond MR image data as input to the trained statistical classifier toobtain corresponding second output; (6) identifying, from the secondoutput, at least one updated location of the at least one landmarkassociated with the at least one midline structure of the patient'sbrain; and (7) determining a degree of change in the midline shift usingthe at least one initial location of the at least one landmark and theat least one updated location of the at least one landmark.

In some embodiments, the at least one landmark associated with the atlast one midline structure of the patient's brain may include ananterior attachment point of the falx cerebri (to the interior table ofthe patient's skull), a posterior attachment point of the falx cerebri,a point on the septum pellucidum. In other embodiments, the at least onelandmark may indicate results of segmentation of the left and rightsides of brain and/or the top and bottom portions of the brain.

In some embodiments, identifying, from the first output of the trainedstatistical classifier, the at least one initial location of the atleast one landmark associated with the at least one midline structure ofthe patient's brain includes: (1) identifying an initial location of ananterior attachment point of the falx cerebri; (2) identifying aninitial location of a posterior attachment point of the falx cerebri;and (3) identifying an initial location of a measurement point on aseptum pellucidum. Identifying, from the second output of the trainedstatistical classifier, the at least one updated location of the atleast one landmark associated with the at least one midline structure ofthe patient's brain includes: (1) identifying an updated location of theanterior attachment point of the falx cerebri; (2) identifying anupdated location of the posterior attachment point of the falx cerebri;and (3) identifying an updated location of the measurement point on theseptum pellucidum. In turn, the degree of change in the midline shiftmay be performed using the identified initial and updated locations ofthe anterior attachment point of the falx cerebri, the posteriorattachment point of the falx cerebri, and the measurement point on theseptum pellucidum.

In some embodiments, determining the degree of change in the midlineshift comprises: determining an initial amount of midline shift usingthe identified initial locations of the anterior attachment point of thefalx cerebri, the posterior attachment point of the falx cerebri, andthe measurement point on the septum pellucidum; determining an updatedamount of midline shift using the identified updated locations of theanterior attachment point of the falx cerebri, the posterior attachmentpoint of the falx cerebri, and the measurement point on the septumpellucidum; and determining the degree of change in the midline shiftusing the determined initial and updated amounts of midline shift.

In some embodiments, the trained statistical classifier may be amulti-layer neural network. For example, the multi-layer neural networkmay be a convolutional neural network (e.g., one having convolutionallayers, pooling layers, and a fully connected layer) or a fullyconvolutional neural network (e.g., a convolutional neural networkwithout a fully connected layer). As another example, the multi-layerneural network may include a convolutional and a recurrent (e.g., longshort-term memory) neural network.

The inventors have also developed low-field MRI techniques fordetermining whether there is a change in the size of an abnormality(e.g., hemorrhage, a lesion, an edema, a stroke core, a stroke penumbra,and/or swelling) in a patient's brain. In some embodiments, low-fieldMRI monitoring techniques may be combined with machine learningtechniques to continuously monitor the size of the abnormality anddetect a change in its size over time. In such embodiments, low-fieldMRI monitoring allows for obtaining a sequence of images of a patient'sbrain and the machine learning techniques (e.g., deep learningtechniques such as convolutional neural networks) may be used todetermine, from the sequence of images, a corresponding sequence ofsizes of the abnormality. For example, the deep learning techniquesdeveloped by the inventors may be used to segment the abnormality in MRIimages, identify points that specify major axes of a 2D or 3D boundingregion (e.g., box), identify maximum diameter of the abnormality and amaximum orthogonal diameter of the abnormality that is orthogonal to themaximum diameter, and/or perform any other processing in furtherance ofidentifying the size of the abnormality.

Accordingly, in some embodiments, abnormality size monitoring involves,while a patient is positioned within a low-field MRI device: (1)acquiring first magnetic resonance (MR) image data for a portion of thepatient's brain; (2) providing the first MR image data as input to atrained statistical classifier (e.g., a multi-layer neural network, aconvolutional neural network, a fully convolutional neural network) toobtain corresponding first output; (3) identifying, using the firstoutput, at least one initial value of at least one feature indicative ofa size of an abnormality in the patient's brain; (4) acquiring second MRimage data for the portion of the patient's brain subsequent toacquiring the first MR image data; (5) providing the second MR imagedata as input to the trained statistical classifier to obtaincorresponding second output; (5) identifying, using the second output,at least one updated value of the at least one feature indicative of thesize of the abnormality in the patient's brain; (6) determining thechange in the size of the abnormality using the at least one initialvalue of the at least one feature and the at least one updated value ofthe at least one feature.

In some embodiments, the at least one initial value of the at least onefeature indicative of the size of the abnormality may include multiplevalues specifying a region surrounding the abnormality (e.g., valuesspecifying a bounding region, values specifying the perimeter of theabnormality, etc.). In some embodiments, the at least one initial valueof the at least one feature may include values specifying one or morediameters of the abnormality (e.g., diameters 1102 and diameter 1104orthogonal to diameter 1102, as shown in FIG. 11A).

In some embodiments, determining the change in the size of theabnormality involves: (1) determining an initial size of the abnormalityusing the at least one value of the at least one feature; (2)determining an updated size of the abnormality using the at least oneupdated value of the at least one feature; and (3) determining thechange in the size of the abnormality using the determined initial andupdated sizes of the abnormality.

Following below are more detailed descriptions of various conceptsrelated to, and embodiments of, methods and apparatus for performingmonitoring using low-field magnetic resonance applications includinglow-field MRI. It should be appreciated that various aspects describedherein may be implemented in any of numerous ways. Examples of specificimplementations are provided herein for illustrative purposes only. Inaddition, the various aspects described in the embodiments below may beused alone or in any combination, and are not limited to thecombinations explicitly described herein.

FIG. 1 is a block diagram of exemplary components of a MRI system 100.In the illustrative example of FIG. 1, MRI system 100 comprisesworkstation 104, controller 106, pulse sequences store 108, powermanagement system 110, and magnetic components 120. It should beappreciated that system 100 is illustrative and that a MRI system mayhave one or more other components of any suitable type in addition to orinstead of the components illustrated in FIG. 1.

As illustrated in FIG. 1, magnetic components 120 comprises B₀ magnet122, shim coils 124, RF transmit and receive coils 126, and gradientcoils 128. B₀ magnet 122 may be used to generate, at least in part, themain magnetic field B₀. B₀ magnet 122 may be any suitable type of magnetthat can generate a main magnetic field (e.g., a low-field strength ofapproximately 0.2 T or less), and may include one or more B₀ coils,correction coils, etc. Shim coils 124 may be used to contribute magneticfield(s) to improve the homogeneity of the B₀ field generated by magnet122. Gradient coils 128 may be arranged to provide gradient fields and,for example, may be arranged to generate gradients in the magnetic fieldin three substantially orthogonal directions (X, Y, Z) to localize whereMR signals are induced.

RF transmit and receive coils 126 may comprise one or more transmitcoils that may be used to generate RF pulses to induce a magnetic fieldB₁. The transmit/receive coil(s) may be configured to generate anysuitable type of RF pulses configured to excite an MR response in asubject and detect the resulting MR signals emitted. RF transmit andreceive coils 126 may include one or multiple transmit coils and one ormultiple receive coils. The configuration of the transmit/receive coilsvaries with implementation and may include a single coil for bothtransmitting and receiving, separate coils for transmitting andreceiving, multiple coils for transmitting and/or receiving, or anycombination to achieve single channel or parallel MRI systems. Thus, thetransmit/receive magnetic component is often referred to as Tx/Rx orTx/Rx coils to generically refer to the various configurations for thetransmit and receive component of an MRI system. Each of magneticscomponents 120 may be constructed in any suitable way. For example, insome embodiments, one or more of magnetics components 120 may befabricated using the laminate techniques described in the aboveincorporated co-filed applications.

Power management system 110 includes electronics to provide operatingpower to one or more components of the low-field MRI system 100. Forexample, power management system 110 may include one or more powersupplies, gradient power amplifiers, transmit coil amplifiers, and/orany other suitable power electronics needed to provide suitableoperating power to energize and operate components of the low-field MRIsystem 100.

As illustrated in FIG. 1, power management system 110 comprises powersupply 112, amplifier(s) 114, transmit/receive switch 116, and thermalmanagement components 118. Power supply 112 includes electronics toprovide operating power to magnetic components 120 of the low-field MRIsystem 100. For example, power supply 112 may include electronics toprovide operating power to one or more B₀ coils (e.g., B₀ magnet 122) toproduce the main magnetic field for the low-field MRI system. In someembodiments, power supply 112 may be a unipolar, continuous wave (CW)power supply, however, any suitable power supply may be used.Transmit/receive switch 116 may be used to select whether RF transmitcoils or RF receive coils are being operated.

Amplifier(s) 114 may include one or more RF receive (Rx) pre-amplifiersthat amplify MR signals detected by one or more RF receive coils (e.g.,coils 124), one or more RF transmit (Tx) amplifiers configured toprovide power to one or more RF transmit coils (e.g., coils 126), one ormore gradient power amplifiers configured to provide power to one ormore gradient coils (e.g., gradient coils 128), shim amplifiersconfigured to provide power to one or more shim coils (e.g., shim coils124).

Thermal management components 118 provide cooling for components oflow-field MRI system 100 and may be configured to do so by facilitatingthe transfer of thermal energy generated by one or more components ofthe low-field MRI system 100 away from those components. Thermalmanagement components 118 may include, without limitation, components toperform water-based or air-based cooling, which may be integrated withor arranged in close proximity to MRI components that generate heatincluding, but not limited to, B₀ coils, gradient coils, shim coils,and/or transmit/receive coils. Thermal management components 118 mayinclude any suitable heat transfer medium including, but not limited to,air and water, to transfer heat away from components of the low-fieldMRI system 100.

As illustrated in FIG. 1, low-field MRI system 100 includes controller106 (also referred to as a console) having control electronics to sendinstructions to and receive information from power management system110. Controller 106 may be configured to implement one or more pulsesequences, which are used to determine the instructions sent to powermanagement system 110 to operate the magnetic components 120 in adesired sequence. For example, controller 106 may be configured tocontrol power management system 110 to operate the magnetic components120 in accordance with a balance steady-state free precession (bSSFP)pulse sequence, a low-field gradient echo pulse sequence, a low-fieldspin echo pulse sequence, a low-field inversion recovery pulse sequence,arterial spin labeling, diffusion weighted imaging (DWI), and/or anyother suitable pulse sequence. Controller 106 may be implemented ashardware, software, or any suitable combination of hardware andsoftware, as aspects of the disclosure provided herein are not limitedin this respect.

In some embodiments, controller 106 may be configured to implement apulse sequence by obtaining information about the pulse sequence frompulse sequences repository 108, which stores information for each of oneor more pulse sequences. Information stored by pulse sequencesrepository 108 for a particular pulse sequence may be any suitableinformation that allows controller 106 to implement the particular pulsesequence. For example, information stored in pulse sequences repository108 for a pulse sequence may include one or more parameters foroperating magnetics components 120 in accordance with the pulse sequence(e.g., parameters for operating the RF transmit and receive coils 126,parameters for operating gradient coils 128, etc.), one or moreparameters for operating power management system 110 in accordance withthe pulse sequence, one or more programs comprising instructions that,when executed by controller 106, cause controller 106 to control system100 to operate in accordance with the pulse sequence, and/or any othersuitable information. Information stored in pulse sequences repository108 may be stored on one or more non-transitory storage media.

As illustrated in FIG. 1, controller 106 also interacts with computingdevice 104 programmed to process received MR data. For example,computing device 104 may process received MR data to generate one ormore MR images using any suitable image reconstruction process(es).Controller 106 may provide information about one or more pulse sequencesto computing device 104 for the processing of data by the computingdevice. For example, controller 106 may provide information about one ormore pulse sequences to computing device 104 and the computing devicemay perform an image reconstruction process based, at least in part, onthe provided information.

Computing device 104 may be any electronic device that may processacquired MR data and generate one or more images of the subject beingimaged. In some embodiments, computing device 104 may be a fixedelectronic device such as a desktop computer, a server, a rack-mountedcomputer, or any other suitable fixed electronic device that may beconfigured to process MR data and generate one or more images of thesubject being imaged. Alternatively, computing device 104 may be aportable device such as a smart phone, a personal digital assistant, alaptop computer, a tablet computer, or any other portable device thatmay be configured to process MR data and generate one or images of thesubject being imaged. In some embodiments, computing device 104 maycomprise multiple computing devices of any suitable type, as the aspectsare not limited in this respect. A user 102 may interact withworkstation 104 to control aspects of the low-field MR system 100 (e.g.,program the system 100 to operate in accordance with a particular pulsesequence, adjust one or more parameters of the system 100, etc.) and/orview images obtained by the low-field MR system 100. According to someembodiments, computing device 104 and controller 106 form a singlecontroller, while in other embodiments, computing device 104 andcontroller 106 each comprise one or more controllers. It should beappreciated that the functionality performed by computing device 104 andcontroller 106 may be distributed in any way over any combination of oneor more controllers, as the aspects are not limited for use with anyparticular implementation or architecture.

FIGS. 2A and 2B illustrate bi-planar magnetic configurations that may beused in a low-field MRI system suitable for use with the changedetection techniques described herein. FIG. 2A schematically illustratesa bi-planar magnet configured to produce, at least in part, a portion ofa B₀ field suitable for low-field MRI. Bi-planar magnet 200 comprisestwo outer coils 210 a and 210 b and two inner coils 212 a and 212 b.When appropriate current is applied to the coils, a magnetic field isgenerated in the direction indicated by the arrow to produce a B₀ fieldhaving a field of view between the coils that, when designed andconstructed appropriately, may be suitable for low-field MRI. The term“coil” is used herein to refer to any conductor or combination ofconductors of any geometry having at least one “turn” that conductscurrent to produce a magnetic field, thereby forming an electromagnet.

It should be appreciated that the bi-planar geometry illustrated in FIG.2A is generally unsuitable for high-field MRI due to the difficulty inobtaining a B₀ field of sufficient homogeneity at high-field strengths.High-field MRI systems typically utilize solenoid geometries (andsuperconducting wires) to achieve the high field strengths of sufficienthomogeneity for high-field MRI. The bi-planar B₀ magnet illustrated inFIG. 2A provides a generally open geometry, facilitating its use withpatients who suffer from claustrophobia and may refuse to be imaged withconventional high-field solenoid coil geometries. Furthermore, thebi-planar design may facilitate use with larger patients as a result ofits open design and, in some instances, a generally larger field of viewpossible at low-field strengths and homogeneity requirements. Moreover,the generally open design facilitates access to the patient being imagedand may improve the ability to position a patient within the field ofview, for example, an unconscious, sedated or anesthetized patient, asdiscussed in further detail below. The bi-planar geometry in FIG. 2A ismerely exemplary, as more or fewer coils may be arranged as needed, asthe aspects are not limited in this respect.

FIG. 2B illustrates a hybrid bi-planar magnet using laminate techniquesto fabricate a B₀ magnet or portion thereof and/or to fabricate one ormore other magnetic components for use in low-field MRI. For example, inthe exemplary bi-planar magnet 200′ illustrated in FIG. 2B, laminatepanels 220 a and 220 b replace inner coils 212 a and 212 b to produce ahybrid magnet. Laminate panels 220 a and 220 b may include any number oflaminate layers having fabricated thereon one or more B₀ coils, gradientcoils, correction coils and/or shim coils, etc. or portions thereof tofacilitate production of the magnetic fields used in low-field MRI.Suitable hybrid magnets using laminate techniques are described in U.S.patent application Ser. No. 14/845,652 ('652 application), filed Sep. 4,2015 and titled “Low Field Magnetic Resonance Imaging Methods andApparatus,” which is herein incorporated by reference in its entirety.In other embodiments, laminate techniques can be used to implement theB₀ magnet in its entirety (e.g., replacing coils 210 a and 210 b).

Exemplary laminate panels 220 a and 220 b may, additionally oralternatively, have fabricated thereon one or more gradient coils, orportions thereof, to encode the spatial location of received MR signalsas a function of frequency or phase. According to some embodiments, alaminate panel comprises at least one conductive layer patterned to formone or more gradient coils, or a portion of one or more gradient coils,capable of producing or contributing to magnetic fields suitable forproviding spatial encoding of detected MR signals when operated in alow-field MRI system. For example, laminate panel 220 a and/or laminatepanel 220 b may comprise a first gradient coil configured to selectivelyvary the B₀ field in a first (X) direction to perform frequency encodingin that direction, a second gradient coil configured to selectively varythe B₀ field in a second (Y) direction substantially orthogonal to thefirst direction to perform phase encoding, and/or a third gradient coilconfigured to selectively vary the B₀ field in a third (Z) directionsubstantially orthogonal to the first and second directions to enableslice selection for volumetric imaging applications.

Exemplary laminate panels 220 a and 220 b may, additionally oralternatively, include additional magnetic components such as one ormore shim coils arranged to generate magnetic fields in support of thesystem to, for example, increase the strength and/or improve thehomogeneity of the B₀ field, counteract deleterious field effects suchas those created by operation of the gradient coils, loading effects ofthe object being imaged, or to otherwise support the magnetics of thelow field MRI system. The bi-planar magnet illustrated in FIGS. 2A and2B, may be produced using conventional coils, laminate techniques, or acombination of both, and may be used to provide magnetic components fora low-field MRI system adapted to perform change detection techniques,as discussed in further detail below.

The inventors have recognized that the low-field context allows for B₀magnet designs not feasible in the high-field regime. For example, dueat least in part to the lower field strengths, superconducting materialand the corresponding cryogenic cooling systems can be eliminated. Duein part to the low-field strengths, B₀ electromagnets constructed usingnon-superconducting material (e.g., copper) may be employed in thelow-field regime. However, such electromagnets still may consumerelatively large amounts of power during operation. For example,operating an electromagnet using a copper conductor to generate amagnetic field of 0.2 T or more requires a dedicated or specializedpower connection (e.g., a dedicated three-phase power connection). Theinventors have developed MRI systems that can be operated using mainselectricity (i.e., standard wall power), allowing the MRI system to bepowered at any location having common power connection, such as astandard wall outlet (e.g., 120V/20 A connection in the U.S.) or commonlarge appliance outlets (e.g., 220-240V/30 A). Thus, a low-power MRIsystem facilitates portability and availability, allowing an MRI systemto be operated at locations where it is needed (e.g., the MRI system canbe brought to the patient instead of vice versa), examples of which arediscussed in further detail below. In addition, operating from standardwall power eliminates the electronics conventionally needed to convertthree-phase power to single-phase power and to smooth out the powerprovided directly from the grid. Instead, wall power can be directlyconverted to DC and distributed to power the components of the MRIsystem.

FIGS. 2C and 2D illustrate a B₀ magnet formed using an electromagnet anda ferromagnetic yoke. In particular, B₀ magnet 2000 is formed in part byan electromagnet 2010 arranged in a bi-planar geometry comprisingelectromagnetic coils 2012 a and 2012 b on an upper side andelectromagnetic coils 2014 a and 2014 b on a lower side of B₀ magnet2000. According to some embodiments, the coils forming electromagnet2010 may be formed from a number of turns of a copper wire or copperribbon, or any other conductive material suitable for producing amagnetic field when operated (e.g., when electrical current is driventhrough the conductor windings). While the exemplary electromagnetillustrated in FIGS. 2C and 2D comprises two pairs of coils, anelectromagnet may be formed using any number of coils in anyconfiguration, as the aspects are not limited in this respect. Theelectromagnetic coils forming electromagnet 2010 may be formed, forexample, by winding a conductor 2013 (e.g., a copper ribbon, wire,paint, etc.) about a fiberglass ring 2017. For example, conductor 2013may be a suitable insulated copper wire, or alternatively, conductor2013 may be a copper ribbon wound in conjunction with an insulatinglayer (e.g., a Mylar layer) to electrically isolate the multiplewindings of the coil. A connector 2019 may be provided to allow for apower connection to provide current to operate coils 2014 a and 2014 bin series. A similar connector on the upper side of the electromagnet(not visible in FIGS. 2C and 2D) may be provided to operate coils 2012 aand 2012 b.

It should be appreciated that the electromagnetic coils may be formedfrom any suitable material and dimensioned in any suitable way so as toproduce or contribute to a desired B₀ magnetic field, as the aspects arenot limited for use with any particular type of electromagnet. As onenon-limiting example that may be suitable to form, in part, anelectromagnet (e.g., electromagnet 2010), an electromagnetic coil may beconstructed using copper ribbon and mylar insulator having 155 turns toform an inner diameter of approximately 23-27 inches (e.g.,approximately 25 inches), an outer diameter of approximately 30-35inches (e.g., 32 inches). However, different material and/or differentdimensions may be used to construct an electromagnetic coil havingdesired characteristics, as the aspects are not limited in this respect.The upper and lower coil(s) may be positioned to provide a distance ofapproximately 10-15 inches (e.g., approximately 12.5 inches) between thelower coil on the upper side and the upper coil on the lower side. Itshould be appreciated that the dimensions will differ depending on thedesired characteristics including, for example, field strength, field ofview, etc.

In the exemplary B₀ magnet illustrated in FIGS. 2C and 2D, each coilpair 2012 and 2014 is separated by thermal management components 2030 aand 2030 b, respectively, to transfer heat produced by theelectromagnetic coils and gradient coils (not illustrated in FIGS. 2Cand 2D) away from the magnets to provide thermal management for the MRIdevice. In particular, thermal management components 2030 a and 2030 bmay comprise a cooling plate having conduits that allow coolant to becirculated through the cooling plate to transfer heat away from themagnets. The cooling plate 2030 a, 2030 b may be constructed to reduceor eliminate eddy currents induced by operating the gradient coils thatcan produce electromagnetic fields that disrupt the B₀ magnetic fieldproduced by the B₀ magnet 2000. For example, thermal managementcomponents 2030 a and 2030 b may be the same or similar to any of thethermal management components described in U.S. application Ser. No.14/846,042 entitled “Thermal Management Methods and Apparatus,” filed onSep. 4, 2015, which is incorporated by reference herein in its entirety.According to some embodiments, thermal management components may beeliminated, as discussed in further detail below.

B₀ magnet 2000 further comprises a yoke 2020 that is magneticallycoupled to the electromagnet to capture magnetic flux that, in theabsence of yoke 2020, would be lost and not contribute to the fluxdensity in the region of interest between the upper and lowerelectromagnetic coils. In particular, yoke 2020 forms a “magneticcircuit” connecting the coils on the upper and lower side of theelectromagnet so as to increase the flux density in the region betweenthe coils, thus increasing the field strength within the imaging region(also referred to as the field of view) of the B₀ magnet. The imagingregion or field of view defines the volume in which the B₀ magneticfield produced by a given B0 magnet is suitable for imaging. Moreparticularly, the imaging region or field of view corresponds to theregion for which the B₀ magnetic field is sufficiently homogeneous at adesired field strength that detectable MR signals are emitted by anobject positioned therein in response to application of radio frequencyexcitation (e.g., a suitable radio frequency pulse sequence). Yoke 2020comprises frame 2022 and plates 2024 a, 2024 b, which may be formedusing any suitable ferromagnetic material (e.g., iron, steel, etc.).Plates 2024 a, 2024 b collect magnetic flux generated by the coil pairsof electromagnet 2010 and directs it to frame 2022 which, in turn,returns the flux back to the opposing coil pair, thereby increasing, byup to a factor of two, the magnetic flux density in the imaging regionbetween the coil pairs (e.g., coil pair 2012 a, 2012 b and coil pair2014 a, 2014 b) for the same amount of operating current provided to thecoils. Thus, yoke 2020 can be used to produce a higher B₀ field(resulting in higher SNR) without a corresponding increase in powerrequirements, or yoke 2020 can be used to lower the power requirementsof B₀ magnet 2000 for a given B₀ field.

According to some embodiments, the material used for portions of yoke2020 (i.e., frame 2022 and/or plates 2024 a, 2024 b) is steel, forexample, a low-carbon steel, silicon steel, cobalt steel, etc. Accordingto some embodiments, gradient coils (not shown in FIGS. 2C, 2D) of theMRI system are arranged in relatively close proximity to plates 2024 a,2024 b inducing eddy currents in the plates. To mitigate, plates 2024 a,2024 b and/or frame 2022 may be constructed of silicon steel, which isgenerally more resistant to eddy current production than, for example,low-carbon steel. It should be appreciated that yoke 2020 may beconstructed using any ferromagnetic material with sufficient magneticpermeability and the individual parts (e.g., frame 2022 and plates 2024a, 2024 b) may be constructed of the same or different ferromagneticmaterial, as the techniques of increasing flux density is not limitedfor use with any particular type of material or combination ofmaterials. Furthermore, it should be appreciated that yoke 2020 can beformed using different geometries and arrangements.

It should be appreciated that the yoke 2020 may be made of any suitablematerial and may be dimensioned to provide desired magnetic flux capturewhile satisfying other design constraints such as weight, cost, magneticproperties, etc. As an example, the frame of the yoke (e.g., frame 2022)may be formed of a low-carbon steel of less than 0.2% carbon or siliconsteel, with the long beam(s) having a length of approximately 38 inches,a width of approximately 8 inches, and a thickness (depth) ofapproximately 2 inches, and the short beam(s) having a length ofapproximately 19 inches, a width of approximately 8 inches and athickness (depth of approximately 2 inches. The plates (e.g., plates2024 a and 2024 b) may be formed from a low-carbon steel of less than0.2% carbon or silicon steel and have a diameter of approximately 30-35inches (e.g., approximately 32 inches). However, the above provideddimensions and materials are merely exemplary of a suitable embodimentof a yoke that can be used to capture magnetic flux generated by anelectromagnet.

As an example of the improvement achieved via the use of yoke 2020,operating electromagnet 2010 to produce a B₀ magnetic field ofapproximately 20 mT without yoke 2020 consumes about 5 kW, whileproducing the same 20 mT B₀ magnetic field with yoke 2020 consumes about750 W of power. Operating electromagnet 2010 with the yoke 2020, a B₀magnetic field of approximately 40 mT may be produced using 2 kW ofpower and a B₀ magnetic field of approximately 50 mT may be producedusing approximately 3 kW of power. Thus, the power requirements can besignificantly reduced by use of yoke 220 allowing for operation of a B₀magnet without a dedicated three-phase power connection. For example,mains electrical power in the United States and most of North America isprovided at 120V and 60 Hz and rated at 15 or 20 amps, permittingutilization for devices operating below 1800 and 2400 W, respectively.Many facilities also have 220-240 VAC outlets with 30 amp ratings,permitting devices operating up to 7200 W to be powered from suchoutlets. According to some embodiments, a low-field MRI system utilizinga B₀ magnet comprising an electromagnet and a yoke (e.g., B₀ magnet2000) is configured to be powered via a standard wall outlet, asdiscussed in further detail below. According to some embodiments, alow-field MRI system utilizing a B₀ magnet comprising an electromagnetand a yoke (e.g., B₀ magnet 2000) is configured to be powered via a220-240 VAC outlet, as also discussed in further detail below.

Referring again to FIGS. 2C and 2D, exemplary B₀ magnet 2010 furthercomprises shim rings 2040 a, 2040 b and shim disks 2042 a, 2042 bconfigured to augment the generated B₀ magnetic field to improvehomogeneity in the field of view (e.g., in the region between the upperand lower coils of the electromagnet where the B₀ field is suitable forsufficient MR signal production), as best seen in FIG. 2D in which thelower coils have been removed. In particular, shim rings 2040 and shimdisk 2042 are dimensioned and arranged to increase the uniformity of themagnetic field generated by the electromagnet at least within the fieldof view of the B₀ magnet. In particular, the height, thickness andmaterial of shim rings 2040 a, 2040 b and the diameter, thickness andmaterial of shim disks 2042 a, 2042 b may be chosen so as to achieve aB₀ field of suitable homogeneity. For example, the shim disk may beprovided with a diameter of approximately 5-6 inches and a width ofapproximately 0.3-0.4 inches. A shim ring may be formed from a pluralityof circular arc segments (e.g., 8 circular arc segments) each having aheight of approximately 20-22 inches, and a width of approximately 2inches to form a ring having an inner diameter of approximately between21-22 inches and approximately between 23-24 inches.

The weight of the B₀ magnet is a significant portion of the overallweight of the MRI system which, in turn, impacts the portability of theMRI system. In embodiments that primarily use low carbon and/or siliconsteel for the yoke and shimming components, an exemplary B₀ magnet 2000dimensioned similar to that described in the foregoing may weighapproximately 550 kilograms. According to some embodiments, cobalt steel(CoFe) may be used as the primary material for the yoke (and possiblythe shim components), potentially reducing the weight of B₀ magnet 2000to approximately 450 Kilograms. However, CoFe is generally moreexpensive than, for example, low carbon steel, driving up the cost ofthe system. Accordingly, in some embodiments, select components may beformed using CoFe to balance the tradeoff between cost and weightarising from its use. Using such exemplary B⁰ magnets a portable,cartable or otherwise transportable MRI system may be constructed, forexample, by integrating the B₀ magnet within a housing, frame or otherbody to which castors, wheels or other means of locomotion can beattached to allow the MRI system to be transported to desired locations(e.g., by manually pushing the MRI system and/or including motorizedassistance). As a result, an MRI system can be brought to the locationin which it is needed, increasing its availability and use as a clinicalinstrument and making available MRI applications that were previouslynot possible.

The primary contributor to the overall power consumption of a low-fieldMRI system employing a B₀ magnet such as B₀ magnet 2000 is theelectromagnet (e.g., electromagnet 2010). For example, in someembodiments, the electromagnet may consume 80% or more of the power ofthe overall MRI system. To significantly reduce the power requirementsof the MRI system, the inventors have developed B₀ magnets that utilizepermanent magnets to produce and/or contribute to the B₀ electromagneticfield. According to some embodiments, B₀ electromagnets are replacedwith permanent magnets as the main source of the B₀ electromagneticfield. A permanent magnet refers to any object or material thatmaintains its own persistent magnetic field once magnetized. Materialsthat can be magnetized to produce a permanent magnet are referred toherein as ferromagnetic and include, as non-limiting examples, iron,nickel, cobalt, neodymium (NdFeB) alloys, samarium cobalt (SmCo) alloys,alnico (AlNiCo) alloys, strontium ferrite, barium ferrite, etc.Permanent magnet material (e.g., magnetizable material that has beendriven to saturation by a magnetizing field) retains its magnetic fieldwhen the driving field is removed. The amount of magnetization retainedby a particular material is referred to as the material's remanence.Thus, once magnetized, a permanent magnet generates a magnetic fieldcorresponding to its remanence, eliminating the need for a power sourceto produce the magnetic field.

FIG. 2E illustrates a permanent B₀ magnet, in accordance with someembodiments. In particular, B₀ magnet 2100 is formed by permanentmagnets 2110 a and 2110 b arranged in a bi-planar geometry and a yoke2120 that captures electromagnetic flux produced by the permanentmagnets and transfers the flux to the opposing permanent magnet toincrease the flux density between permanent magnets 2110 a and 2110 b.Each of permanent magnets 2110 a and 2110 b are formed from a pluralityof concentric permanent magnets. In particular, as visible in FIG. 2E,permanent magnetic 2110 b comprises an outer ring of permanent magnets2114 a, a middle ring of permanent magnets 2114 b, an inner ring ofpermanent magnets 2114 c, and a permanent magnet disk 2114 d at thecenter. Permanent magnet 2110 a may comprise the same set of permanentmagnet elements as permanent magnet 2110 b.

The permanent magnet material used may be selected depending on thedesign requirements of the system. For example, according to someembodiments, the permanent magnets (or some portion thereof) may be madeof NdFeB, which produces a magnetic field with a relatively highmagnetic field per unit volume of material once magnetized. According tosome embodiments, SmCo material is used to form the permanent magnets,or some portion thereof. While NdFeB produces higher field strengths(and in general is less expensive than SmCo), SmCo exhibits less thermaldrift and thus provides a more stable magnetic field in the face oftemperature fluctuations. Other types of permanent magnet material(s)may be used as well, as the aspects are not limited in this respect. Ingeneral, the type or types of permanent magnet material utilized willdepend, at least in part, on the field strength, temperature stability,weight, cost and/or ease of use requirements of a given B₀ magnetimplementation.

The permanent magnet rings are sized and arranged to produce ahomogenous field of a desired strength in the central region (field ofview) between permanent magnets 2110 a and 2110 b. In the exemplaryembodiment illustrated in FIG. 2E, each permanent magnet ring comprisesa plurality segments, each segment formed using a plurality of blocksthat are stacked in the radial direction and positioned adjacent to oneanother about the periphery to form the respective ring. The inventorshave appreciated that by varying the width (in the direction tangent tothe ring) of each permanent magnet, less waste of useful space may beachieved while using less material. For example, the space betweenstacks that does not produce useful magnetic fields can be reduced byvarying the width of the blocks, for example, as function of the radialposition of the block, allowing for a closer fit to reduce wasted spaceand maximize the amount of magnetic field that can be generated in agiven space. The dimensions of the blocks may also be varied in anydesired way to facilitate the production of a magnetic field of desiredstrength and homogeneity, as discussed in further detail below.

B₀ magnet 2100 further comprises yoke 2120 configured and arranged tocapture magnetic flux generated by permanent magnets 2110 a and 2110 band direct it to the opposing side of the B₀ magnet to increase the fluxdensity in between permanent magnets 2110 a and 2110 b, increasing thefield strength within the field of view of the B₀ magnet. By capturingmagnetic flux and directing it to the region between permanent magnets2110 a and 2110 b, less permanent magnet material can be used to achievea desired field strength, thus reducing the size, weight and cost of theB₀ magnet. Alternatively, for given permanent magnets, the fieldstrength can be increased, thus improving the SNR of the system withouthaving to use increased amounts of permanent magnet material. Forexemplary B₀ magnet 2100, yoke 2120 comprises a frame 2122 and plates2124 a and 2124 b. In a manner similar to that described above inconnection with yoke 2020, plates 2124 a and 2124 b capture magneticflux generated by permanent magnets 2110 a and 2110 b and direct it toframe 2122 to be circulated via the magnetic return path of the yoke toincrease the flux density in the field of view of the B₀ magnet. Yoke2120 may be constructed of any desired ferromagnetic material, forexample, low carbon steel, CoFe and/or silicon steel, etc. to providethe desired magnetic properties for the yoke. According to someembodiments, plates 2124 a and 2124 b (and/or frame 2122 or portionsthereof) may be constructed of silicon steel or the like in areas wherethe gradient coils could most prevalently induce eddy currents.

Exemplary frame 2122 comprises arms 2123 a and 2123 b that attach toplates 2124 a and 2124 b, respectively, and supports 2125 a and 2125 bproviding the magnetic return path for the flux generated by thepermanent magnets. The arms are generally designed to reduce the amountof material needed to support the permanent magnets while providingsufficient cross-section for the return path for the magnetic fluxgenerated by the permanent magnets. Arms 2123 a has two supports withina magnetic return path for the B₀ field produced by the B₀ magnet.Supports 2125 a and 2125 b are produced with a gap 2127 formed between,providing a measure of stability to the frame and/or lightness to thestructure while providing sufficient cross-section for the magnetic fluxgenerated by the permanent magnets. For example, the cross-sectionneeded for the return path of the magnetic flux can be divided betweenthe two support structures, thus providing a sufficient return pathwhile increasing the structural integrity of the frame. It should beappreciated that additional supports may be added to the structure, asthe technique is not limited for use with only two supports and anyparticular number of multiple support structures.

As discussed above, exemplary permanent magnets 2110 a and 2110 bcomprise a plurality of rings of permanent magnetic materialconcentrically arranged with a permanent magnet disk at the center. Eachring may comprise a plurality of stacks of ferromagnetic material toform the respective ring, and each stack may include one or more blocks,which may have any number (including a single block in some embodimentsand/or in some of the rings). The blocks forming each ring may bedimensioned and arranged to produce a desired magnetic field. Theinventors have recognized that the blocks may be dimensioned in a numberof ways to decrease cost, reduce weight and/or improve the homogeneityof the magnetic field produced, as discussed in further detail inconnection with the exemplary rings that together form permanent magnetsof a B₀ magnet, in accordance with some embodiments.

FIG. 2F illustrates a B₀ magnet 2200, in accordance with someembodiments. B⁰ magnet 2200 may share design components with B₀ magnet2100 illustrated in FIG. 2E. In particular, B₀ magnet 2200 is formed bypermanent magnets 2210 a and 2210 b arranged in a bi-planar geometrywith a yoke 2220 coupled thereto to capture electromagnetic fluxproduced by the permanent magnets and transfer the flux to the opposingpermanent magnet to increase the flux density between permanent magnets2210 a and 2210 b. Each of permanent magnets 2210 a and 2210 b areformed from a plurality of concentric permanent magnets, as shown bypermanent magnet 2210 b comprising an outer ring of permanent magnets2214 a, a middle ring of permanent magnets 2214 b, an inner ring ofpermanent magnets 2214 c, and a permanent magnet disk 2214 d at thecenter. Permanent magnet 2210 a may comprise the same set of permanentmagnet elements as permanent magnet 2210 b. The permanent magnetmaterial used may be selected depending on the design requirements ofthe system (e.g., NdFeB, SmCo, etc. depending on the propertiesdesired).

The permanent magnet rings are sized and arranged to produce ahomogenous field of a desired strength in the central region (field ofview) between permanent magnets 2210 a and 2210 b. In particular, in theexemplary embodiment illustrated in FIG. 2F, each permanent magnet ringcomprises a plurality of circular arc segments sized and positioned toproduce a desired B₀ magnetic field, as discussed in further detailbelow. In a similar manner to yoke 2120 illustrated in FIG. 2E, yoke2220 is configured and arranged to capture magnetic flux generated bypermanent magnets 2210 a and 2210 b and direct it to the opposing sideof the B₀ magnet to increase the flux density in between permanentmagnets 2210 a and 2210 b. Yoke 2220 thereby increases the fieldstrength within the field of view of the B₀ magnet with less permanentmagnet material, reducing the size, weight and cost of the B₀ magnet.Yoke 2220 also comprises a frame 2222 and plates 2224 a and 2224 b that,in a manner similar to that described above in connection with yoke2220, captures and circulates magnetic flux generated by the permanentmagnets 2210 a and via the magnetic return path of the yoke to increasethe flux density in the field of view of the B₀ magnet. The structure ofyoke 2220 may be similar to that described above to provide sufficientmaterial to accommodate the magnetic flux generated by the permanentmagnets and providing sufficient stability, while minimizing the amountof material used to, for example, reduce the cost and weight of the B₀magnet.

Because a permanent B₀ magnet, once magnetized, will produce its ownpersistent magnetic field, power is not required to operate thepermanent B₀ magnet to generate its magnetic field. As a result, asignificant (often dominant) contributor to the overall powerconsumption of an MRI system can be eliminated, facilitating thedevelopment of an MRI system that can be powered using mains electricity(e.g., via a standard wall outlet or common large household applianceoutlets). As discussed above, the inventors have developed low power,portable low-field MRI systems that can be deployed in virtually anyenvironment and that can be brought to the patient who will undergo animaging procedure. In this way, patients in emergency rooms, intensivecare units, operating rooms and a host of other locations can benefitfrom MRI in circumstances where MRI is conventionally unavailable.

FIGS. 3A-3B illustrate a portable or cartable low-field MRI system 300suitable for use in performing change detection techniques describedherein, in accordance with some embodiments. System 300 may includemagnetic and power components, and potentially other components (e.g.,thermal management, console, etc.), arranged together on a singlegenerally transportable and transformable structure. System 300 may bedesigned to have at least two configurations; a configuration adaptedfor transport and storage, and a configuration adapted for operation.FIG. 3A shows system 300 when secured for transport and/or storage andFIG. 3B shows system 300 when transformed for operation. System 300comprises a portion 390A that can be slid into and retracted from aportion 390B when transforming the system from its transportconfiguration to its operation configuration, as indicated by the arrowsshown in FIG. 3B. Portion 390A may house power electronics, console(which may comprise an interface device such as a touch panel display)and thermal management. Portion 390A may also include other componentsused to operate system 300 as needed.

Portion 390B comprises magnetic components of low-field MRI system 300,including laminate panels on which magnetic components are integrated inany of the combinations discussed herein. When transformed to theconfiguration adapted for operating the system to perform MRI (as shownin FIG. 3B), supporting surfaces of portions 390A and 390B provide asurface on which the patient can lie. A slide-able bed or surface 384may be provided to facilitate sliding the patient into position so thata portion of the patient to be imaged is within the field of view of thelow-field MRI magnetic components. System 300 provides for a portablecompact configuration of a low-field MRI system that facilitates accessto MRI imaging in circumstances where it conventionally is not available(e.g., in the NICU).

FIGS. 3A-3B illustrate an example of a convertible low-field MRI systemthat utilizes a bi-planar magnet forming and imaging region betweenhousings 386A and 386B. Housings 386A and 386B house magnetic componentsfor the convertible system 300. According to some embodiments, themagnetic components may be produced, manufactured and arranged usingexclusively laminate techniques, exclusively traditional techniques, orusing a combination of both (e.g., using hybrid techniques). Theconvertible low-field MRI system 300 allows the system to be brought tothe patient to facilitate monitoring of target anatomy of the patient.For example, convertible low-field MRI system 300 may be brought to apatient in the NICU and the unconscious patient may be placed on theslide-able bed and positioned within the field of view of the system.The patient may then be monitored by obtaining continuous, periodicand/or regular MRI images over an extended period of time (e.g., overthe course of one or multiple hours) to evaluate changes taking placesusing any of the various change detection techniques described herein.

FIGS. 3C and 3D illustrate views of another portable MRI system 3800,which may be used to implement various change detection techniques, inaccordance with some embodiments of the technology described herein.Portable MRI system 3800 comprises a B₀ magnet 3810 formed in part by anupper magnet 3810 a and a lower magnet 3810 b having a yoke 3820 coupledthereto to increase the flux density within the imaging region. The B₀magnet 3810 may be housed in magnet housing 3812 along with gradientcoils 3815 (e.g., any of the gradient coils described in U.S.application Ser. No. 14/845,652, titled “Low Field Magnetic ResonanceImaging Methods and Apparatus” and filed on Sep. 4, 2015, which isherein incorporated by reference in its entirety). According to someembodiments, B₀ magnet 3810 comprises an electromagnet. According tosome embodiments, B₀ magnet 3810 comprises a permanent magnet (e.g., anypermanent magnet described in U.S. application Ser. No. 15/640,369,titled “LOW-FIELD MAGNETIC RESONANCE IMAGING METHODS AND APPARATUS,”filed on Jun. 30, 2017, which is incorporated by reference herein in itsentirety).

Portable MRI system 3800 further comprises a base 3850 housing theelectronics needed to operate the MRI system. For example, base 3850 mayhouse electronics including, but not limited to, one or more gradientpower amplifiers, an on-system computer, a power distribution unit(PDU), one or more power supplies, and/or any other power componentsconfigured to operate the MRI system using mains electricity (e.g., viaa connection to a standard wall outlet and/or a large appliance outlet).For example, base 3870 may house low power components, such as thosedescribed herein, enabling at least in part the portable MRI system tobe powered from readily available wall outlets. Accordingly, portableMRI system 3800 can be brought to the patient and plugged into a walloutlet in the vicinity.

Portable MRI system 3800 further comprises moveable slides 3860 that canbe opened and closed and positioned in a variety of configurations.Slides 3860 include electromagnetic shielding 3865, which can be madefrom any suitable conductive or magnetic material, to form a moveableshield to attenuate electromagnetic noise in the operating environmentof the portable MRI system to shield the imaging region from at leastsome electromagnetic noise. As used herein, the term electromagneticshielding refers to conductive or magnetic material configured toattenuate the electromagnetic field in a spectrum of interest andpositioned or arranged to shield a space, object and/or component ofinterest. In the context of an MRI system, electromagnetic shielding maybe used to shield electronic components (e.g., power components, cables,etc.) of the MRI system, to shield the imaging region (e.g., the fieldof view) of the MRI system, or both.

The degree of attenuation achieved from electromagnetic shieldingdepends on a number of factors including the type material used, thematerial thickness, the frequency spectrum for which electromagneticshielding is desired or required, the size and shape of apertures in theelectromagnetic shielding (e.g., the size of the spaces in a conductivemesh, the size of unshielded portions or gaps in the shielding, etc.)and/or the orientation of apertures relative to an incidentelectromagnetic field. Thus, electromagnetic shielding refers generallyto any conductive or magnetic barrier that acts to attenuate at leastsome electromagnetic radiation and that is positioned to at leastpartially shield a given space, object or component by attenuating theat least some electromagnetic radiation.

It should be appreciated that the frequency spectrum for which shielding(attenuation of an electromagnetic field) is desired may differdepending on what is being shielded. For example, electromagneticshielding for certain electronic components may be configured toattenuate different frequencies than electromagnetic shielding for theimaging region of the MRI system. Regarding the imaging region, thespectrum of interest includes frequencies which influence, impact and/ordegrade the ability of the MRI system to excite and detect an MRresponse. In general, the spectrum of interest for the imaging region ofan MRI system correspond to the frequencies about the nominal operatingfrequency (i.e., the Larmor frequency) at a given B₀ magnetic fieldstrength for which the receive system is configured to or capable ofdetecting. This spectrum is referred to herein as the operating spectrumfor the MRI system. Thus, electromagnetic shielding that providesshielding for the operating spectrum refers to conductive or magneticmaterial arranged or positioned to attenuate frequencies at least withinthe operating spectrum for at least a portion of an imaging region ofthe MRI system.

In portable MRI system 3800 illustrated in FIGS. 3C and 3D, the moveableshields are thus configurable to provide shielding in differentarrangements, which can be adjusted as needed to accommodate a patient,provide access to a patient, and/or in accordance with a given imagingprotocol. For example, for the imaging procedure illustrated in FIG. 3E(e.g., a brain scan), once the patient has been positioned, slides 3960can be closed, for example, using handle 3862 to provide electromagneticshielding 3965 around the imaging region except for the opening thataccommodates the patient's upper torso. In the imaging procedureillustrated in FIG. 3F (e.g., a scan of the knee), slides 3960 may bearranged to have openings on both sides to accommodate the patient'slegs. Accordingly, moveable shields allow the shielding to be configuredin arrangements suitable for the imaging procedure and to facilitatepositioning the patient appropriately within the imaging region.

In some embodiments, a noise reduction system comprising one or morenoise reduction and/or compensation techniques may be performed tosuppress at least some of the electromagnetic noise that is not blockedor sufficiently attenuated by shielding 3865. In particular, theinventors have developed noise reduction systems configured to suppress,avoid and/or reject electromagnetic noise in the operating environmentin which the MRI system is located. According to some embodiments, thesenoise suppression techniques work in conjunction with the moveableshields to facilitate operation in the various shielding configurationsin which the slides may be arranged. For example, when slides 3960 arearranged as illustrated in FIG. 3F, increased levels of electromagneticnoise will likely enter the imaging region via the openings. As aresult, the noise suppression component will detect increasedelectromagnetic noise levels and adapt the noise suppression and/oravoidance response accordingly. Due to the dynamic nature of the noisesuppression and/or avoidance techniques described herein, the noisereduction system is configured to be responsive to changing noiseconditions, including those resulting from different arrangements of themoveable shields. Thus, a noise reduction system in accordance with someembodiments may be configured to operate in concert with the moveableshields to suppress electromagnetic noise in the operating environmentof the MRI system in any of the shielding configurations that may beutilized, including configurations that are substantially withoutshielding (e.g., configurations without moveable shields).

To ensure that the moveable shields provide shielding regardless of thearrangements in which the slides are placed, electrical gaskets may bearranged to provide continuous shielding along the periphery of themoveable shield. For example, as shown in FIG. 3D, electrical gaskets3867 a and 3867 b may be provided at the interface between slides 3860and magnet housing to maintain to provide continuous shielding alongthis interface. According to some embodiments, the electrical gasketsare beryllium fingers or beryllium-copper fingers, or the like (e.g.,aluminum gaskets), that maintain electrical connection between shields3865 and ground during and after slides 3860 are moved to desiredpositions about the imaging region. According to some embodiments,electrical gaskets 3867 c are provided at the interface between slides3860, as illustrated in FIG. 3F so that continuous shielding is providedbetween slides in arrangements in which the slides are brought together.Accordingly, moveable slides 3860 can provide configurable shielding forthe portable MRI system.

To facilitate transportation, a motorized component 3880 is provide toallow portable MRI system to be driven from location to location, forexample, using a control such as a joystick or other control mechanismprovided on or remote from the MRI system. In this manner, portable MRIsystem 3800 can be transported to the patient and maneuvered to thebedside to perform imaging, as illustrated in FIGS. 3E and 3F. Asdiscussed above, FIG. 3E illustrates a portable MRI system 3900 that hasbeen transported to a patient's bedside to perform a brain scan. FIG. 3Fillustrates portable MRI system 3900 that has been transported to apatient's bedside to perform a scan of the patient's knee.

The portable MRI systems described herein may be operated from aportable electronic device, such as a notepad, tablet, smartphone, etc.For example, tablet computer 3875 may be used to operate portable MRIsystem to run desired imaging protocols and to view the resultingimages. Tablet computer 3875 may be connected to a secure cloud totransfer images for data sharing, telemedicine, and/or deep learning onthe data sets. Any of the techniques of utilizing network connectivitydescribed in U.S. application Ser. No. 14/846,158, titled “AutomaticConfiguration of a Low Field Magnetic Resonance Imaging System,” filedSep. 4, 2015, which is herein incorporated by reference in its entirety,may be utilized in connection with the portable MRI systems describedherein.

FIG. 3G illustrates another example of a portable MRI system, inaccordance with some embodiments of the technology described herein.Portable MRI system 4000 may be similar in many respects to portable MRIsystems illustrated in FIGS. 3C-3F. However, slides 4060 are constructeddifferently, as is shielding 4065, resulting in electromagnetic shieldsthat are easier and less expensive to manufacture. As discussed above, anoise reduction system may be used to allow operation of a portable MRIsystem in unshielded rooms and with varying degrees of shielding aboutthe imaging region on the system itself, including no, or substantiallyno, device-level electromagnetic shields for the imaging region.

It should be appreciated that the electromagnetic shields illustrated inFIGS. 3C-3G are exemplary and providing shielding for an MRI system isnot limited to the example electromagnetic shielding described herein.Electromagnetic shielding can be implemented in any suitable way usingany suitable materials. For example, electromagnetic shielding may beformed using conductive meshes, fabrics, etc. that can provide amoveable “curtain” to shield the imaging region. Electromagneticshielding may be formed using one or more conductive straps (e.g., oneor more strips of conducting material) coupled to the MRI system aseither a fixed, moveable or configurable component to shield the imagingregion from electromagnetic interference, some examples of which aredescribed in further detail below. Electromagnetic shielding may beprovided by embedding materials in doors, slides, or any moveable orfixed portion of the housing. Electromagnetic shields may be deployed asfixed or moveable components, as the aspects are not limited in thisrespect.

FIG. 4 illustrates a method of monitoring a patient using low-field MRIto detect changes therein, in accordance with some embodiments. In act410, first MR image data is acquired by a low-field MRI device of atarget portion of anatomy (e.g., a portion of the brain, a portion of aknee, etc.) of a patient positioned within the low-field MRI device.Positioning a patient within the low-field device refers to placing thepatient relative to the magnetic components of the low-field MRI devicesuch that a portion of the patient's anatomy is located within the fieldof view of the low-field MRI device so that MR image data can beacquired. The term MR image data is used herein to refer to MR datagenerically including, but not limited to, MR data prior to imagereconstruction (e.g., k-space MR data) and MR data that has beenprocessed in some way (e.g., post-image reconstruction MR data such as athree dimensional (3D) volumetric image). Because both registration andchange detection techniques described herein can be performed in anydomain (or a combination of domains), the term MR image data is used torefer to acquired MR data agnostic to domain and/or whether imagereconstruction (or any other processing) has been performed. As anexample application, MR image data of a patient's brain may be acquiredto monitor temporal changes within the brain (e.g., changes regarding ananeurysm or bleeding within the brain, changes in a tumor or othertissue anomaly, changes in chemical composition, etc.).

In act 420, subsequent (next) MR image data is acquired of the same orsubstantially the same portion of the anatomy included in the first MRimage data. The next MR image data may be acquired immediately followingacquisition of the first MR image data, or may be obtained after adesired period of delay (e.g., after 1, 2, 3, 4, 5, 10, 15, 20 minutes,etc.). As a result, the next MR image data captures the portion of theanatomy after some finite amount of time has elapsed. The inventors haveappreciated that low-field MRI facilitates relatively fast imageacquisition, allowing a temporal sequence of MR image data to beacquired in relatively quick succession, thus capturing changes that maybe of interest to the physician. The accessibility, availability and/orrelative low cost of the low-field MRI system enables MR data to beacquired over extended periods of time at any time interval needed tomonitor and/or otherwise observe and evaluate the patient.

As with the first MR image data, the next MR image data may be of anyform (e.g., a 3D volumetric image, a 2D image, k-space MR data, etc.).According to some embodiments, the next MR image data (or any subsequentnext MR image data acquired) is obtained using the same acquisitionparameters used to acquire the first MR image data. For example, thesame pulse sequence, field of view, SNR, and resolution may be used toacquire MR signals from the same portion of the patient. In this manner,the MR image data may be compared to evaluate changes that have occurredwithin the anatomy being imaged. For example, as described below, MRimage data may be used to determine whether there is a change in thedegree of midline shift in a patient. As another example, as describedbelow, MR image data may be used to determine whether there is a changein a size of an abnormality (e.g., a hemorrhage, a lesion, an edema, astroke core, a stroke penumbra, and/or swelling) in a patient. In otherembodiments, one or more acquisition parameters may be altered to changethe acquisition strategy for acquiring next MR image data, as discussedin further detail below in connection with FIG. 5.

The first, next and any subsequent MR image data acquired are referredto as respective “frames” of MR image data. A sequence of frames may beacquired and the individual frames may be registered in a sequence offrames acquired over time. Thus, a frame corresponds to acquired MRimage data representative of the particular time at which the MR imagedata was acquired. Frames need not include the same amount of MR imagedata or correspond to the same field of view, but frames generally needsufficient overlap so that adequate feature descriptors can be detected(e.g., sufficient subject matter in common between frames).

In act 430, the first and next MR image data are co-registered oraligned with one another. Any suitable technique may be used toco-register the first and next MR image data, or any pair of acquired MRimage data for which change detection processing is desired. In thesimplest case, registration may be performed by assuming that thepatient is still so that the MR image data is aligned withouttransforming or deforming the MR image data. However, such a simplifiedtechnique does not account for movement of the patient, changesresulting from breathing, etc., which may need to be compensated for inother ways to avoid attributing observed differences between imagesresulting from these factors to biological processes. More sophisticatedregistration techniques used to align the MR image data to account formovement of the patient, breathing, etc., include, but are not limitedto, the use of deformation models and/or correlation techniques adaptedto MR image data acquired at different points in time.

According to some embodiments, co-registering acquired MR image datainvolves determining a transformation that best aligns the MR image data(e.g., in a least squares sense). The transformation between MR imagedata acquired at different points in time may include translation,rotation, scale or any suitable linear or non-linear deformation, as theaspects are not limited in this respect. The transformation may bedetermined at any desired scale. For example, a transformation may bedetermined for a number of identified sub-regions (e.g., volumesincluding a number of voxels) of the MR image data, or may be determinedfor each voxel in the MR image data. The transformation may bedetermined in any manner, for example, using a deformation model thatdeforms a mesh or coordinate frame of first MR image data to thecoordinate frame of next MR image data and vice versa. Any suitableregistration technique may be used, as the aspects are not limited inthis respect. An illustrative process for co-registering MR image dataacquired at different points in time in accordance with some embodiment,is discussed in further detail below in connection with FIG. 6.

In act 440, one or more changes are detected in the co-registered MRimage data. For example, once the MR image data has been co-registered,differences between the MR image data can be attributed to changes inthe patient's anatomy being imaged (e.g., morphological changes to theanatomy or other changes to the biology or physiology of the imagedanatomy), such as a change in the size of an aneurysm, increased ordecreased bleeding, progression or regression of a tumor or other tissueanomaly, changes in chemical composition, or other biological orphysiological changes of interest. Change detection can be performed inany suitable way. For example, once the MR image data has beenco-registered, change detection may be performed in k-space usingamplitude and phase information (coherent change detection), or changedetection can be performed in the image domain using intensityinformation (non-coherent change detection). Generally speaking,coherent change detection may be more sensitive, revealing changes onthe sub-voxel level. However, even though non-coherent change detectionmay be generally less sensitive, change detection in the image domainmay be more robust to co-registration errors.

In some embodiments, change detection may be performed by derivingfeatures from each MR frame in a sequence of MR frames and comparing thefeatures to one another. For example, in some embodiments, imageprocessing techniques (e.g., including the deep learning techniquesdescribed herein) may be applied to each MR frame in a sequence of twoor more MR frames, obtained by imaging a patient's brain, to identify arespective sequence of two or more midline shift measurements. In turn,the sequence of midline shift measurements may be used to determinewhether there is a change in the degree of midline shift for the patientbeing monitored. As another example, in some embodiments, imageprocessing techniques (e.g., the deep learning techniques describedherein), may be applied to each MR frame in a sequence of two or more MRframes, obtained by imaging a patient's brain, to identify a respectivesequence of two or more measurements of a size of an abnormality in thepatient's brain (e.g., a hemorrhage, a lesion, an edema, a stroke core,a stroke penumbra, and/or swelling). In turn, the sequence of sizemeasurements may be used to determine whether there is a change in thesize of the abnormality in the brain of a patient being monitored.

In some embodiments, multi-resolution techniques may be used to performchange detection. For example, the first MR image data may correspond toa baseline high-resolution image, and subsequently-acquired MR imagedata may correspond to low-resolution images that may be correlated withthe high-resolution baseline image. Acquiring low-resolution images mayspeed up the frame rate of the change detection process enabling theacquisition of more data in a shorter period of time. Any suitabletechniques or criteria may be used to determine which data to acquirefor a low-resolution image. The particular data to acquire for alow-resolution image may be determined using, for example, wavelets,selective k-space sampling, polyphase filtering, key-frame basedtechniques, etc. Sparse sampling of k-space over short time intervals(e.g., time-varying selective sampling of k-space), as an example,results in better time resolution.

The selection of particular data to acquire may also be determined bydetecting changes between MR image data frames. For example, when achange is detected, a 1D or 2D volume selection having a field of viewthat includes the location of the detected change may be selected foracquisition to interrogate a particular part of the anatomydemonstrating change over time.

Using coherent change detection, differences in phase and amplitude ineach “frame” of acquired MR data are evaluated. For example,co-registered frames of MR image data may be subtracted to obtaindifference information indicative of changes occurring in the MR data.According to some embodiments, a finite impulse response (FIR) filter isapplied to each “voxel” in the frame, which can be used as a reference.Filtering can also be used to provide a “look-ahead filter” thatconsiders a number of frames over which to perform change detection. Forexample, a current, previous and next frame may be evaluated using asliding window to analyze changes over a desired number of frames.

The inventors have recognized that acquiring a full 3D volume of MR datamay take a substantial amount of time. In some embodiments, changedetection is used to selectively determine particular data (e.g.,particular lines in k-space) to acquire, such that MR data used forimage reconstruction may be acquired in a shorter timeframe than wouldbe required to acquire a full 3D volume. For example, using the slidingwindow approach described above, an initial 3D volume may first beacquired. Then, at subsequent points in time, rather than reacquiringthe full 3D volume, a subset of the lines in k-space selected based onparts of the image that are changing may be acquired and the previous 3Dvolume may be updated with the newly acquired data.

In some embodiments, a particular feature or area of interest may beidentified a priori, and the acquisition sequence may be tailored toacquire lines of k-space that will emphasize the identified feature orarea of interest. For example, the acquisition sequence may focus onacquiring just the edges of k-space or any other suitable part ofk-space. In some embodiments, the identified area of interest may be aportion of the anatomy. For example, to analyze a post-surgical bleed,it may not be necessary to acquire data on the entire anatomy. Rather,select portions of k-space that correspond to the anatomy of interestfor monitoring may be sampled multiple times in a relatively briefperiod of time to enable a physician to closely monitor changes in theanatomy of interest over the shorter timescale providing for a hightemporal correlation between the acquisitions.

Using non-coherent change detection, the intensity of voxels in 3Dimages reconstructed from acquired MR data may be compared to evaluatechanges as they occur over time. Detected changes, either evaluatedcoherently (e.g., in k-space) or non-coherently (e.g., in 3D images) maybe conveyed in any number of ways. For example, changes in the MR imagedata may be emphasized on displayed images to provide a visualindication to a physician of changes occurring over time. For example,voxels undergoing change can be rendered in color that in turn may becoded according to the extent of the change that occurred. In thismanner, a physician can quickly see the “hot spots” that are undergoingsignificant change. Alternatively, or in addition to, change detectioncan be performed by analyzing regions over which changes are occurring.For example, connected component analysis may be used to locatecontiguous regions where voxel changes have occurred. That is, regionsof connected voxels that have undergone change may be emphasized ordisplayed differently (e.g., using color, shading, etc.) to indicatethat changes are occurring in the corresponding regions. Changesdetected in acquired MR image data may be conveyed in other ways, as theaspects are not limited in this respect.

Shape and volume analysis may also be performed to assess whether agiven feature of the anatomy of interest is changing (e.g., growing orshrinking, progressing or regressing, or to otherwise characterizechange in the features). For example, image processing techniques can beused to segment MR image data into regions and to assess one or moreproperties of the segment such as shape, volume, etc. Changes to the oneor more segments properties may be conveyed to a physician via a displayor otherwise. For example, the size of a tumor may be monitored across asequence of images to evaluate whether the tumor is increasing ordecreasing in size. As another example, a brain bleed may be monitoredover time wherein the important change to evaluate is the volume of thebleed. Thus, acquired MR image data may be processed to segment featuresof interest (e.g., tumor, bleed, hemorrhage, etc.) and compute thevolume of the corresponding feature.

It should be appreciated that segmented volumes can be analyzed in otherways to characterize metrics of interest for the segmented volume. Forexample, 2D and/or 3D shape descriptors may be applied to the segmentedfeatures to characterize any number of aspects or properties of thesegmented feature including, but not limited to, volume, surface area,symmetry, “texture,” etc. In this way, change detection may be performedon features of interest captured in the acquired MR data to evaluate howthe features are changing over time. Changes detected in segmentedfeatures can be utilized not only to understand how the feature isevolving in time, but characteristics of the particular features can becompared to stored information to assist in differentiating healthy fromunhealthy, normal from anomalous and/or to assess the danger of aparticular condition. The information obtained from the MR data may alsobe stored along with existing information to grow the repository ofinformation that can be used for subsequent data analysis.

According to some embodiments, techniques may be used to remove changesin the data caused by regular or periodic movement, such as breathing orheart beat etc. By determining which parts of the image are changing andwhich are not, it is possible to focus acquisition on only the parts ofthe image that are changing and not acquire data for the parts of theimage that are not changing. By acquiring a smaller set of data onlyrelated to the parts of the image that are changing, the acquisitiontime is compressed. Additionally, some changes in the image are causedby periodic events such as breathing and heartbeats. In someembodiments, periodic events are modeled based on their periodicity toenable a change detection process to ignore the periodic movementscaused by the period events when determining which parts of the imageare changing and should be the focus of acquisition.

According to some embodiments, change detection may be performed bydetecting the rate of change of MR image data over a sequence ofacquired MR image data. As used herein, a rate of change refers to anyfunctional form of time. Detecting the rate of change may provide richerdata regarding the subject matter being imaged, such as indicating theseverity of a bleed, size of a hemorrhage, increase in midline shift,the aggressiveness of a lesion, etc. As one example, when a contrastagent is administered, there is a natural and expected way in which thecontrast agent is taken up by the body. The uptake of contrast agent isdetected as a signal increase that will register as a change having aparticular functional form. The manner in which the signal changes asthe contrast agent washes out and/or is metabolized will also give riseto a detectable change in signal that will have a functional form overtime. The functional form of changes over time can provide informationabout the type, aggressiveness or other characteristics of a lesion orother abnormality that can provide clinically useful and/or criticaldata. As another example, a stroke victim may be monitored after astroke has occurred, changes in the time course of the stroke lesionthat differs from expected might be used to alert personnel to unusualchanges, provide a measure of drug efficacy, or provide otherinformation relevant to the condition of the patient. In general,detecting rate of change can facilitate higher order analysis of thesubject matter being imaged.

Techniques are available that facilitate faster acquisition of MR data,enabling quicker image acquisition for low-field MRI. For example,compressed sensing techniques, sparse imaging array techniques and MRfingerprinting are some examples of techniques that can expedite MRimage acquisition. Additionally, in some embodiments, Doppler techniquesmay be used to analyze multiple frames of images over a short period oftime to estimate velocities that may be used to filter out parts of theimage that are not changing.

Upon completion of detecting changes in acquired MR image data, act 420may be repeated to obtain further MR image data, either immediately orafter waiting for a predetermined amount of time before acquiringsubsequent MR image data. Subsequently acquired MR image data may becompared with any MR image data previously acquired to detect changesthat have occurred over any desired interval of time (e.g., by repeatingact 430 and 440). In this manner, sequences of MR image data can beobtained and changes detected and conveyed to facilitate understandingof the temporal changes taking place in the portion of the anatomy ofthe patient being monitored, observed and/or evaluated. It should beappreciated that any acquired MR image data can be registered andanalyzed for change. For example, successive MR image data may becompared so that, for example, changes on a relatively small time scalecan be detected. The detected change may be conveyed to a physician sothat the anatomy of interest can be continuously, regularly and/orperiodically monitored.

In addition, acquired MR image data may be stored so that a physiciancan request change detection be performed at desired points of interest.For example, a physician may be interested to see changes that havetaken place within the last hour and may specify that change detectionbe performed between MR image data acquired an hour ago and present timeMR image data. The physician may specify an interval of time, mayspecify multiple times of interest, or may select thumbnails oftimestamped images to indicate which MR image data the physician wouldlike change detection performed. Thus, the techniques described hereinmay be used to monitor ongoing changes and/or to evaluate changes thathave occurred over any interval of time during which MR image data hasbeen acquired. The above described change detection techniques may beused to enable monitoring, evaluation and observation of a patient overa period of time, thus enabling MRI to be utilized as a monitoring toolin ways that conventional MRI and other modalities cannot be used.

In some embodiments, acquired MR image data may be used to evaluatechange with respect to a stored high-field MRI scan. In this way, apatient may be imaged using a high-field MRI scan initially, butsubsequent monitoring (which would not be feasible using high-field MRI)would be performed using a low-field MRI system, examples of which areprovided herein. The change detection techniques described herein can beapplied not only to detecting changes between sets MR image dataacquired by a low-field MRI system, but also to detecting changesbetween MR image data acquired by a high-field MRI system (e.g.,initially) and MR image data acquired by a low-field MRI system (e.g.,subsequently), regardless of the order in which the high-field MR imagedata and the low-field MR image data was obtained.

FIG. 5 illustrates a method of changing an acquisition strategy based,at least in part, on observations made regarding change detection. Theinventors have developed a multi-acquisition console that allowsacquisition parameters to be modified on the fly to dynamically updatean acquisition strategy implemented by the low-field MRI system. Forexample, commands to the low-field MRI system can be streamed from theconsole to achieve dynamic updates to the acquisition process. Theinventors have appreciated that the ability to dynamically updateacquisition parameters and/or change the acquisition strategy can beexploited to achieve a new paradigm for MRI, enabling the MRI system tobe used for monitoring a patient and adapting the acquisition strategybased on observations of the acquired MR image data (e.g., based onchange detection information).

In method 500 illustrated in FIG. 5, acts 510-540 may be similar to acts410-440 of method 400 illustrated in FIG. 4 to obtain change detectioninformation in regard to MR image data obtained by a low-field MRIsystem. In act 550, at least one acquisition parameter may be updated,changed or other modified based on the results of change detection.Acquisition parameters that may be varied are not limited in anyrespect, and may include any one or combination of field of view,signal-to-noise ratio (SNR), resolution, pulse sequence type, etc. Someexamples of acquisition parameters that may be changed are described infurther detail below.

According to some embodiments, change detection information may be usedto update the acquisition parameters to, for example, increase SNR of MRdata obtained from a particular region. For example, based oncharacteristics of co-registration (e.g., properties of thetransformation, deformation models, etc.) and/or changes observed inparticular regions, it may be desirable to increase the SNR in thoseregions to, for example, better evaluate the subject matter present, toimprove further change detection, or otherwise obtaining moreinformation regarding the portion of the anatomy being monitored and/orobserved. Similarly, acquisition parameters may be altered to obtainhigher resolution MR data for particular regions of the portion ofanatomy being monitored/observed. Change detection may reveal that apatient has moved or subject matter of interest is no longer optimallyin the field of view. This information may be utilized to dynamicallychange the field of view of subsequent image acquisition.

According to some embodiments, the type of pulse sequence that isapplied may be changed based on what is observed in change detectiondata obtained from acquired MR image data. Different pulse sequences maybe better at capturing particular types of information and thesedifferences can be exploited to allow for appropriate exploration basedon observed change detection data. Due, at least in part, to the dynamiccapability of the system developed by the inventors, different pulsesequences can be interleaved, alternated or otherwise utilized toacquire MR data that captures information of interest. For example, afast spin echo sequence may have been used to acquire a number of framesof MR image data and the results of change detection may suggest thebenefit of changing to a different pulse sequence, for example, a bSSFPsequence to observe a particular change (e.g., to obtain different MRdata, to allow for higher SNR or resolution in a particular region,etc.). In this manner, changes that may not be observable using one typeof sequence may be seen by changing the type of pulse sequence beingused.

As another example, pulse sequences may be chosen for the type ofcontrast provided (e.g., T1, T2, etc.) or the type of information thatis captured, and the appropriate pulse sequence can be utilized toobtain MR data, which can be changed dynamically during the monitoringprocess. The choice of pulse sequence or combination of pulse sequencesused can be guided by the change detection information that is obtained.For example, MR data may be captured using a given pulse sequence and,based on obtained change detection information (e.g., based oninformation obtained by performing act 540), the pulse sequence may bechanged to explore a region using magnetic resonance spectroscopy (MRS).In this manner, exploration of the chemical composition of a portion ofanatomy being monitored may be initiated as a result of changes observedin the MR data.

It should be appreciated that the acquisition parameters may be varieddynamically at any time during acquisition. That is, a full acquisitionneed not complete before altering the acquisition strategy. As a result,updating acquisition parameter(s) may be performed based on partialacquisition and/or partial image reconstruction to facilitate anacquisition strategy that is fully dynamic. The ability to dynamicallyupdate any one or combination of acquisition parameters allows MRI to beutilized as a monitoring and exploration tool, whereas conventional MRIsystems cannot be used in this way.

Some applications, such as diffusion weighted imaging (DWI), requiresubstantial amounts of power due to the higher gradient fields neededfor such applications. In some embodiments, power savings may beachieved by interleaving acquisitions for a DWI (or other) sequence withacquisitions that require less power. By allowing for the dynamic updateof acquisition parameters during an acquisition, any combination andinterleaving of acquisition sequences to achieve a desired goal (e.g.,low power consumption, reduced heating, reducing stress on the gradientcoils, etc.) may be realized.

In some embodiments, biological or physiological events that unfold overa relatively short timeframe may be studied using the change detectiontechniques described herein. For example, for arterial spin labeling, afull data set may be initially obtained, and subsequent acquisitions maysparsely sample the data. Perfusion of the blood over time may bemonitored change detection, where the changes in the image correspond tothe inflowing blood to a particular region of the imaged anatomy.

As discussed, above, co-registration of MR image data acquired atdifferent points in time enables the identification of changes in MRdata by reducing the effect of patient movement on the change detectionprocess. The co-registration may be accomplished with a model for theeffects of deformation. The deformation mesh captures changes in shapeand distribution over time, which may occur from subtle movements of thepatient or from biological morphology. To maintain registration acrossframes as the imaged volume moves or deforms, the k-space acquisitionstrategy may be updated based on new constraints of the deformed volume.For example, acquisition parameters affecting field of view, SNR,resolution, etc., may be updated based on new constraints of thedeformed volume.

FIG. 6 illustrates a technique 600 for co-registering frames of MR imagedata, in accordance with some embodiments. For example, registrationtechnique 600 may be used to align a pair of frames acquired at twoseparate times. In act 610, one or more feature descriptors appearing orcommon to the frames being co-registered are detected. Featuredescriptors may be any feature present in the MR image between framesthat can be reliably detected. Features may include localcharacteristics such as edges, corners, ridges, etc. and/or may includeregion characteristics such as curves, contours, shape, intensitydistributions and/or patterns, etc. Any feature or characteristic thatcan be reliably detected between frames may be used as a featuredescriptor, as the aspects are not limited in this respect. Any suitabletechnique may be used to determine the feature descriptors including,but not limited to, SIFT, SURF, U-SURF, CenSurE, BRIEF, ORB, and cornerdetector techniques such as FAST, Harris, Hessian, and Shi-Tomasi.

After the feature descriptors between frames have been determined, theprocess proceeds to act 620, where associated sub-regions across theframes are correlated. The correlation calculations between sub-regionsmay be performed in any number of dimensions (e.g., 1D, 2D, 3D), asaspects are not limited in this respect. After the correlations betweensub-regions are determined, the process proceeds to act 630, where thewarped or deformed model from frame to frame is determined based on thecorrelations between the sub-regions in the different frames. Once thedeformation of the model is determined between frames, the processproceeds to act 640, wherein the model deformation is used toco-register the data across the multiple frames.

Once the data is co-registered, change detection metrics including, butnot limited to those discussed above, such as coherent changes,non-coherent changes, and others including position changes, velocity,acceleration or time derivative vectors may be determined using theco-registered data. Other metrics including segmentation and geometricshape descriptors such as surface area, volume, crinkliness, sphericalharmonic basis coefficients, etc. may also be determined based on theco-registered data and optionally the metrics may be used to updateacquisition parameters for future acquisitions on the fly as discussedabove.

As described above, the inventors have developed techniques for usinglow-field MRI for monitoring a patient to determine whether there is achange in a degree of midline shift in the patient's brain. Midlineshift refers to an amount of displacement of the brain's midline fromits normal symmetric position due to trauma (e.g., stroke, hemorrhage,or other injury) and is an important indicator for clinicians of theseverity of the brain trauma. The midline shift may be characterized asa shift of the brain past its midline, usually in the direction awayfrom the affected side (e.g., a side with an injury).

In some embodiments, the midline shift may be measured as the distancebetween a midline structure of the brain (e.g., a point on the septumpellucidum) and a line designated as the midline. The midline may becoplanar with the falx cerebri (also known as the cerebral falx), whicha crescent-shaped fold of the meningeal layer of dura mater thatdescends vertically in the longitudinal fissure between the cerebralhemispheres of the human brain. The midline may be represented as a lineconnecting the anterior and posterior attachments of the falx cerebri tothe inner table of the skull.

As one example, illustrated in FIG. 7A, the midline 702 is a lineconnecting the anterior and posterior attachment points 706 a and 706 bof the falx cerebri. In this example, the midline shift may be measuredas the distance between the measurement point 706 c in the septumpellucidum and the midline 702. That distance is the length of the line704 defined by endpoints 706 c and 706 d, and which is orthogonal tomidline 702.

As another example, illustrated in FIG. 7B, the midline 712 is a lineconnecting the anterior and posterior attachment points 716 a and 716 bof the falx cerebri. In this example, the midline shift may be measuredas the distance between the measurement point 716 c in the septumpellucidum and the midline 712. That distance is the length of the line714 defined by endpoints 716 c and 716 d, and which is orthogonal tomidline 712.

FIG. 8 is a flowchart of an illustrative process 800 for determining adegree of change in the midline shift of a patient, in accordance withsome embodiments of the technology described herein. In someembodiments, the entirety of process 800 may be performed while thepatient is within a low-field MRI device, which may be of any suitabletype described herein including, for example, any of the low-field MRIdevices illustrated in FIGS. 3A-3G).

Process 800 begins at act 802, where the low-field MRI device acquiresinitial magnetic resonance data of a target portion of the patient'sbrain. As described herein, the term MR image data is used herein torefer to MR data generically including, but not limited to, MR dataprior to image reconstruction (e.g., k-space MR data) and MR data thathas been processed in some way (e.g., post-image reconstruction MR datasuch as a three dimensional (3D) volumetric image). In some embodiments,the initial MR data may include one or more two-dimensional images ofrespective brain slices (e.g., two, three, four, five, etc. neighboringslices). When multiple slices are included, the slices may beneighboring. For example, the initial MR data may include one or more 2Dimages of one or more respective slices in which the two lateralventricles are prominent.

Next, at act 804, the initial MR image data is provided as input to atrained statistical classifier in order to obtain corresponding initialoutput. In some embodiments, prior to being provided to the trainedstatistical classifier, the initial MR image data may be pre-processed,for example, by resampling, interpolation, affine transformation, and/orusing any other suitable pre-processing techniques, as aspects of thetechnology described herein are not limited in this respect.

In some embodiments, the output of the trained statistical classifiermay indicate one or more initial locations, in the initial MR data, ofone or more landmarks associated with at least one midline structure ofthe patient's brain. This location or locations may be identified fromoutput of the trained statistical classifier at act 806 of process 800.The output may specify the location(s) directly or indirectly. In thelatter case, the location(s) may be derived from information included inthe output of the trained statistical classifier.

For example, in some embodiments, the output of the trained statisticalclassifier may indicate the locations of the anterior and posterior falxcerebri attachment points and the location of a measurement point in theseptum pellucidum. When the initial MR data includes a 2D image of acorresponding slice, the output of the trained statistical classifiermay indicate the locations of the landmarks (e.g., falx cerebriattachment points and measurement point in the septum pellucidum) withinthe 2D image. As described above, the locations of the falx cerebriattachment points and the measurement point in the septum pellucidum maybe used to make a midline shift measurement.

In some embodiments the trained statistical classifier may be a neuralnetwork statistical classifier. For example, the training statisticalclassifier may include a convolutional neural network (e.g., asillustrated in FIGS. 9A and 9B), a convolutional neural network and arecurrent neural network, such as a long short-term memory network,(e.g., as illustrated in FIGS. 9A and 9C), a fully convolutional neuralnetwork (e.g., as illustrated in FIG. 10), and/or any other suitabletype of neural network. The trained statistical classifier may beimplemented in software, in hardware, or using any suitable combinationof software and hardware. In some embodiments, one or more machinelearning software libraries may be used to implement the trainedstatistical classifier including, but not limited to, Theano, Torch,Caffe, Keras, and TensorFlow. These libraries may be used for training astatistical classifier (e.g., a neural network) and/or using a trainedstatistical classifier. Aspects of training the trained statisticalclassifier used at acts 804 and 806 are described in more detail below.It should also be appreciated that the trained statistical classifier isnot limited to being a neural network and may be any other suitable typeof statistical classifier (e.g., a support vector machine, a graphicalmodel, a Bayesian classifier, a decision tree classifier, etc.), asaspects of the technology described herein are not limited in thisrespect.

As discussed above, in some embodiments, the trained statisticalclassifier may be a convolutional neural network. FIGS. 9A and 9B showan illustrative example of such a convolutional neural network. As shownin FIG. 9A, an input image (a 256×256 image in this example) is providedas input to the convolutional neural network, which processes the inputimage through an alternating series of convolutional and pooling layers.In this example, the convolutional neural network processes the inputimage using two convolutional layers to obtain 32 256×256 feature maps.Next, after an application of a pooling layer (e.g., a max poolinglayer), two more convolutional layers are applied to obtain 64 128×128feature maps. Next, after an application of another pooling layer (e.g.,max pooling), two more convolutional layers are applied to obtain 12864×64 feature maps. Next, after application of another pooling layer andanother convolutional layer, the resulting 256 32×32 feature maps areprovided as input to the portion of the neural network shown in FIG. 9B.In this portion, after an additional convolutions, the feature maps areprocessed through at least one fully connected layer to generatepredictions. The predictions may, in some embodiments, indicatelocations of falx cerebri attachment points (e.g., posterior andanterior attachment points, and a measurement point on the septumpellucidum).

FIGS. 9A and 9C show another illustrative example of a neural networkthat may be used as the trained statistical classifier, in someembodiments. The neural network of FIGSS. 9A and 9C has a convolutionalneural network portion (shown in FIG. 9A, which was described above) anda recurrent neural network portion (shown in FIG. 9C), which may be usedto model temporal constraints among input images provided as inputs tothe neural network over time. The recurrent neural network portion maybe implemented as a long short-term memory (LSTM) neural network. Such aneural network architecture may be used to process a series of imagesobtained by a low-field MRI apparatus during performance of a monitoringtask. A series of images obtained by the low-field MRI apparatus may beprovided as inputs to the CNN-LSTM neural network, within which,features derived from at least one earlier-obtained image may becombined with features obtained from a later-obtained image to generatepredictions.

In some embodiments, the neural networks illustrated in FIGS. 9A-9C mayuse a kernel size of 3 with a stride of 1 for convolutional layers, akernel size of “2” for pooling layers, and a variance scalinginitializer.

In some embodiments, the neural networks illustrated in FIGS. 9A-C maybe used to process a single image (e.g., a single slice) at a time. Inother embodiments, the neural networks illustrated in FIGS. 9A-9C may beused to process multiple slices (e.g., multiple neighboring slices) atthe same time. In this way, the features used for prediction pointlocations (e.g., locations of the falx cerebri attachment points and ameasurement point on the septum pellucidum) may be computed usinginformation from a single slice or from multiple neighboring slices.

In some embodiments, when multiple slices are being processed by theneural network, the convolutions may be two-dimensional (2D) orthree-dimensional (3D) convolutions. In some embodiments, the processingmay be slice based so that features are calculated for each slice usinginformation from the slice and one or more of its neighboring slices(only from the slice itself or from the slice itself and one or more ofits neighbors). In other embodiments, the processing may be a fully-3Dprocessing pipeline such that features for multiple slices are computedconcurrently using data present in all of the slices.

In some embodiments, rather than using a convolutional neural networkarchitecture with one or more fully connected output layers, as shown inFIGS. 9A-9C, a fully-convolutional neural network architecture may beemployed. In such an architecture, the output is a single-channel outputhaving the same dimensionality as the input. In this approach, a map ofpoint locations (e.g., falx cerebri attachment points) is created byintroducing Gaussian kernel intensity profiles at point locations, withthe neural network trained to regress these profiles using mean-squarederror loss.

FIG. 10 illustrates two different fully convolutional neural networkarchitectures, which may be used in some embodiments. The firstarchitecture, with processing involving processing path (a), includesthree portions: (1) an output compressive portion comprising a series ofalternating convolutional and pooling layers; (2) a long short-termmemory portion (indicated by path (a)); and (3) an input expandingportion comprising a series of alternating convolutional anddeconvolutional layers. This type of architecture may be used to modeltemporal constraints, as can the neural network architecture of FIGS. 9Aand 9 c. The second architecture, with processing involving processingpath (b), includes three portions: (1) an output compressive portioncomprising a series of alternating convolutional and pooling layers; (2)a convolutional network portion (indicated by path (b)); and (3) aninput expanding portion comprising a series of alternating convolutionaland deconvolutional layers and a center-of-mass layer. The center ofmass layer computes the estimate as a center of mass computed from theregressed location estimates at each location.

In some embodiments, the neural networks illustrated in FIG. 10 may usea kernel size of 3 for convolutional layers with stride of 1, a kernelsize of “2” for the pooling layers, a kernel of size 6 with stride 2 fordeconvolutional layers, and a variance scaling initializer. In someembodiments, when multiple slices are being processed by one of theneural networks shown in FIG. 10, the convolutions may betwo-dimensional (2D) or three-dimensional (3D) convolutions. In someembodiments, the processing may be slice based so that features arecalculated for each slice using information from the slice and one ormore of its neighboring slices. In other embodiments, the processing maybe a fully 3D processing pipeline such that features for multiple slicesare computed concurrently using data present in all of the slices.

It should be appreciated that the neural network architecturesillustrated in FIGS. 9A-9C and FIG. 10 are illustrative and thatvariations of these architectures are possible. For example, one or moreother neural network layers (e.g., a convolutional layer, adeconvolutional layer, a rectified linear unit layer, an upsamplinglayer, a concatenate layer, a pad layer, etc.) may be introduced to anyof the neural network architectures of FIGS. 9A-9C and 10 as anadditional one or more layers and/or instead of one or more layers partof the illustrated architectures. As another example, the dimensionalityof one or more layers may be varied and/or the kernel size for one ormore convolutional, pooling, and/or deconvolutional layers may bevaried.

Next, process 800 proceeds to act 808, where the next MR image data isacquired. The next MR image data is acquired after the initial MR dataacquired. Thus, although, in some embodiments, acts 804 and 806 may beperformed after act 808 is performed, act 808 is generally performedafter act 802. The next MR image data may be acquired immediatelyfollowing acquisition of the initial MR image data, or may be obtainedafter a desired period of delay (e.g., within 1, 2, 3, 4, 5, 10, 15, 20minutes, within one hour, within two hours, etc.). As with the initialMR image data, the next MR image data may be of any form (e.g., a 3Dvolumetric image, a 2D image, k-space MR data, etc.). In someembodiments, the initial MR data and the next MR image data are of thesame type. For example, each of the initial and next MR data may includeone or more two-dimensional images of one or more respective (e.g.,neighboring) brain slices. For example, the initial MR data may includemultiple images of neighboring slices obtained at a first time and thenext MR data may include multiple images of the same neighboring slicesobtained at a second time later than the first time.

Next, process 800 proceeds to act 810 where the next MR image data isprovided as input to the trained statistical classifier to obtain thecorresponding next output. In some embodiments, prior to being providedto the trained statistical classifier, the next MR image data may bepre-processed, for example, by resampling, interpolation, affinetransformation, and/or using any other suitable pre-processingtechniques, as aspects of the technology described herein are notlimited in this respect. The next MR image data may be preprocessed inthe same way as the initial MR data was preprocessed.

In some embodiments, the next output of the trained statisticalclassifier may indicate one or more updated locations, in the next MRdata, of one or more landmarks associated with at least one midlinestructure of the patient's brain. This location or locations may beidentified from output of the trained statistical classifier at act 812of process 800. The output may specify the location(s) directly orindirectly. In the latter case, the location(s) may be derived frominformation included in the output of the trained statisticalclassifier.

For example, in some embodiments, the output of the trained statisticalclassifier obtained at act 812 may indicate the updated locations of theanterior and posterior falx cerebri attachment points and the updatedlocation of a measurement point in the septum pellucidum. When the nextMR data includes a 2D image of a corresponding slice, the correspondingoutput of the trained statistical classifier may indicate the updatedlocations of the landmarks (e.g., falx cerebri attachment points andmeasurement point in the septum pellucidum) within the 2D image. Asdescribed above, the updated locations of the falx cerebri attachmentpoints and the measurement point in the septum pellucidum may be used tomake a new/updated midline shift measurement.

Next, process 800 proceeds to act 814, where the degree of change in themidline shift is determined using the initial and updated locations oflandmarks associated with midline structures that were obtained at acts806 and 812, respectively. For example, in some embodiments, the initiallocations of the falx cerebri attachment points and the measurementpoint in the septum pellucidum may be used to determine (e.g.,calculate) an initial midline shift amount. The updated locations of thefalx cerebri attachment points and the measurement point in the septumpellucidum may be used to determine an updated midline shift amount. Theinitial and updated midline shift amounts may be used to determine(e.g., by evaluating their difference) the degree of change in midlineshift of the patient over the time period between the acquisition ofinitial and next MR data.

Next, process 800 proceeds to decision block 816, where it is determinedwhether to perform a new determination of the degree of change in themidline shift. This determination may be performed in any suitable way(e.g., by determining whether a threshold number of iterations have beenperformed, based on a schedule, based on manual input provided by aclinician, etc.), as aspects of the technology described herein are notlimited in this respect. When it is determined that a new determinationof the degree of change in the midline shift is to be performed, thenprocess 800 returns to block 808 and acts 808-814 are repeated again(with newly obtained MR data being compared to the most recentlypreviously obtained MR data). On the other hand, when it is determinedthat a new determination of the degree of change in the midline shift isnot to be performed, process 800 completes.

It should be appreciated that process 800 is illustrative and that thereare variations. For example, in some embodiments, the trainedstatistical classifier may be trained, as a multi-task model, such thatits output may be used not only to identify one or more locationsassociated with at least one midline structure of the patient's brain,but also to segment the ventricles. As described herein, the measurementpoint to compare on to the midline lies on the septum pellucidum and itis therefore beneficial to use lateral ventricle labels to train amulti-task model, as such a model will identify the location of theseptum pellucidum more accurately. The symmetry or asymmetry of thesegmented lateral ventricles may help to identify the location of theseptum pellucidum more accurately. Such a model may be trained if thetraining data includes lateral ventricle labels in addition to labels ofthe measurement point on the septum pellucidum and the falx cerebriattachment points.

The trained statistical classifier may be trained in any suitable way.In embodiments, where the trained statistical classifier is a neuralnetwork, the neural network may be trained any suitable neural networktraining technique including, but not limited to, gradient descent,stochastic gradient descent, backpropagation, and/or any other suitableiterative optimization technique. In embodiments where the neuralnetwork comprises a recurrent neural network, the training technique mayemploy stochastic gradient descent and backpropagation through time.

In some embodiments, the trained statistical classifier may be trainedusing training data comprising labeled scans of patients. For example,the classifier may be trained using training data comprising labeledscans of patients exhibiting midline shift (e.g., stroke patients and/orcancer patients). The scans may be annotated manually by one or moreclinical experts. In some embodiments, the annotations may includeindications of the locations of the falx cerebri attachment points andmeasurement points on the septum pellucidum. In some embodiments, theannotations may include a line representing the midline (instead of orin addition to indications of the locations of the falx cerebri locationpoints). If there is no midline shift in a particular scan, noindication of the midline (a line or attachment points) may be provided.

The inventors have appreciated that there is an inherent ambiguity ofthe location of the measurement point. Specifically, slight shifts ofthe measurement point along the septum pellucidum may be tolerated, butshifts of the measurement point perpendicular to the pellucidum are notallowed. Accordingly, in some embodiments, the training data may beaugmented by generating additional allowed locations for the location ofthe measurement point along the septum pellucidum.

As described above, the inventors have also developed low-field MRItechniques for determining whether there is a change in the size of anabnormality (e.g., a hemorrhage, a lesion, an edema, a stroke core, astroke penumbra, and/or swelling) in a patient's brain. Indeed, MRI isan important and accurate modality for detecting acute hemorrhage inpatients presenting with acute focal stroke symptoms, and is moreaccurate that CT scans for the detection of chronic intracerebralhemorrhages. Some studies have identified that MRI imaging is betterthan CT imaging for detection of acute ischemia and can accuratelydetect acute and chronic hemorrhage. As a result, MRI may be a preferredimaging modality for accurate diagnosis of patients suspected of havingacute stroke and for monitoring abnormalities associated with a stroke.

Accordingly, in some embodiments, low-field MRI monitoring techniquesmay be combined with machine learning techniques to continuously monitorthe size of the abnormality and detect changes in its size over time. Insuch embodiments, low-field MRI monitoring allows for obtaining asequence of images of a patient's brain and the machine learningtechniques described herein (e.g., deep learning techniques such asconvolutional neural networks) may be used to determine, from thesequence of images, a corresponding sequence of sizes of theabnormality. For example, the deep learning techniques developed by theinventors may be used to segment (e.g., identify the outlines of)hemorrhages in MRI images, identify points that specify major axes of a2D or 3D bounding region (e.g., box), identify a maximum diameter of thehemorrhage and a maximum orthogonal diameter of the hemorrhage that isorthogonal to the maximum diameter, and/or perform any other processingin furtherance of identifying the size of the hemorrhage.

In some embodiments, the volume of an abnormality may be identifiedusing the so-called “ABC/2” formula for spherical or ellipsoidalabnormalities. The value A represents the length of a maximum diameterof the abnormality (e.g. length of diameter 1102 shown in FIG. 11A), thevalue B represents the length of a maximum orthogonal diameter of theabnormality that is orthogonal to the maximum diameter (e.g., length ofdiameter 1104 shown in FIG. 11A), and the value C is the total number ofslices with the abnormality seen in the vertical plane multiplied byslice thickness. The values A, B, and C may then be multiplied and theproduct may be divided by 2 in order to estimate the volume of theabnormality. It should be appreciated that the length of the maximumdiameter “A” and the length of the maximum orthogonal diameter “B” maybe used to estimate the size (e.g., volume) of an abnormality in anyother suitable way, as aspects of the technology described herein arenot limited in this respect.

Accordingly, in some embodiments, the machine learning techniquesdescribed herein may be applied to processing MRI images to identify,within the MRI images, a first maximum diameter of an abnormality and asecond maximum diameter. The first and second maximum diameters in turnmay be used to estimate the size of the abnormality using the ABC/2technique or in any other suitable way. For example, as shown in FIG.11B, the machine learning techniques described herein are used toidentify the first diameter 1106 of an abnormality and the seconddiameter 1108 of the abnormality orthogonal to the first diameter. Thelengths of diameters 1106 and 1108 may be used to estimate the size ofthe abnormality shown in FIG. 11B (a right intraparenchymal basalganglia hemorrhage).

As another example, shown in FIG. 11C, the machine learning techniquesdescribed herein are used to identify the first diameter 1110 of thehematoma and the second diameter 1112 of the hematoma orthogonal to thefirst diameter. The lengths of diameters 1110 and 1112 may be used toestimate the size of the hematoma shown in FIG. 11C (a rightparietotemporal intraparenchymal hematoma). As another example, shown inFIG. 11D, the machine learning techniques described herein are used toidentify the first diameter 1114 of the hemorrhage and the seconddiameter 1116 of the hematoma orthogonal to the first diameter. Thelengths of diameters 1114 and 1116 may be used to estimate the size ofthe hematoma shown in FIG. 11D (a right parietotemporal intraparenchymalhematoma). As another example, shown in FIG. 11E, the machine learningtechniques described herein are used to identify the first diameter 1118of the hemorrhage and the second diameter 1118 of the hemorrhageorthogonal to the first diameter. The lengths of diameters 1118 and 1120may be used to estimate the size of the hemorrhage shown in FIG. 11E(intraparenchymal hemorrhage in the right parietal lobe with mildsurrounding edema). As another example, shown in FIG. 11F, the machinelearning techniques described herein are used to identify the firstdiameter 1122 of the hemorrhage and the second diameter 1124 of thehemorrhage orthogonal to the first diameter. The lengths of diameters1122 and 1124 may be used to estimate the size of the hemorrhage shownin FIG. 11F (hemorrhagic contusions in the frontal lobe).

In some embodiments, changes in the size of an abnormality may bemonitored. The size of the abnormality (e.g., a hemorrhage, a lesion, anedema, a stroke core, a stroke penumbra, and/or swelling) may bemonitored by identifying the size of the abnormality in a series ofimages taken at different times. For example, as shown in FIG. 12A, thesize of the hemorrhage in a first MRI image obtained at a first time maybe determined based on the lengths of the diameters 1202 and 1204, whichare identified using the machine learning techniques described herein(e.g., using a neural network having an architecture illustrated in FIG.14 or FIG. 15). As shown in FIG. 12B, the size of the hemorrhage in asecond MRI image obtained at a second time (occurring at least athreshold amount of time after the first time) may be determined basedon the lengths of the diameters 1206 and 1208, which are also identifiedusing the machine learning techniques described herein. Comparing thelengths of the diameters (and/or the hemorrhage sizes derivedtherefrom), as shown in FIG. 12C, allows one to determine whether thesize of the hemorrhage changed (e.g., did it get smaller or larger?)and, if so, the amount by which the size changed.

FIG. 13 is a flowchart of an illustrative process 1300 for determining adegree of change in the size of an abnormality (e.g., a hemorrhage, alesion, an edema, a stroke core, a stroke penumbra, and/or swelling) ina patient's brain, in accordance with some embodiments of the technologydescribed herein. In some embodiments, the entirety of process 1300 maybe performed while the patient is within a low-field MRI device, whichmay be of any suitable type described herein including, for example, anyof the low-field MRI devices illustrated in FIGS. 3A-3G). Although, forclarity, process 1300 is described with respect to detecting a change inthe size of a hemorrhage, it should be appreciated that process 1300 maybe applied to detecting changes in the size of any suitable type ofabnormality (e.g., a hemorrhage, a lesion, an edema, a stroke core, astroke penumbra, and/or swelling), as aspects of the technologydescribed herein are not limited in this respect. Similarly, the neuralnetwork architectures described in FIGS. 14 and 15 may be applied todetecting changes in the size of any suitable type of abnormality, theyare not limited to being used solely for detecting changes in size of ahemorrhage.

Process 1300 begins at act 1302, where the low-field MRI device acquiresinitial magnetic resonance data of a target portion of the patient'sbrain. As described herein, the term MR image data is used herein torefer to MR data generically including, but not limited to, MR dataprior to image reconstruction (e.g., k-space MR data) and MR data thathas been processed in some way (e.g., post-image reconstruction MR datasuch as a three dimensional (3D) volumetric image). In some embodiments,the initial MR data may include one or more two-dimensional images ofrespective brain slices (e.g., two, three, four, five, etc. neighboringslices). When multiple slices are included, the slices may beneighboring.

Next, at act 1304, the initial MR image data is provided as input to atrained statistical classifier in order to obtain corresponding initialoutput. In some embodiments, prior to being provided to the trainedstatistical classifier, the initial MR image data may be pre-processed,for example, by resampling, interpolation, affine transformation, and/orusing any other suitable pre-processing techniques, as aspects of thetechnology described herein are not limited in this respect.

In some embodiments, the output of the trained statistical classifiermay be used to identify, at act 1306, initial value(s) of feature(s)indicative of the size of a hemorrhage in the patient's brain. In someembodiments, the features may be a first maximum diameter of thehemorrhage in a first direction and a second maximum diameter of thehemorrhage in a second direction, which is orthogonal to the firstdirection. The values may indicate the initial lengths of the diametersand/or the initial endpoints of the diameters (from which the initiallengths may be derived). In some embodiments, the features may becorners of a bounding box bounding the perimeter of the hemorrhage andthe initial values may be the locations of the corners. In someembodiments, the features may specify the boundary of the hemorrhage andthe initial values may be the locations of one or more points along thesegmented boundary. The output of the trained statistical classifier mayspecify the initial value(s) directly or indirectly. In the latter case,the value(s) may be derived from information included in the output ofthe trained statistical classifier.

In some embodiments, initial value(s) of the feature(s) obtained at act1306 may be used to obtain an initial estimate of the size of thehemorrhage. For example, when the initial values may be used todetermine initial lengths of maximum orthogonal diameters of thehemorrhage, the initial lengths may be used to estimate the initialvolume of the hemorrhage (e.g., according to above-described ABC/2method). As another example, when the initial values specify theboundary of a hemorrhage, the boundary information may be used toestimate the initial area of the hemorrhage in the slice (e.g., using apolygonal approximation or in any other suitable way).

In some embodiments the trained statistical classifier may be a neuralnetwork statistical classifier. For example, the training statisticalclassifier may include a fully convolutional neural network (e.g., asillustrated in FIGS. 10 and 14) or a convolutional neural network (e.g.,as illustrated in FIGS. 9A-9C and 15), and/or any other suitable type ofneural network. The trained statistical classifier may be implemented insoftware, in hardware, or using any suitable combination of software andhardware. In some embodiments, one or more machine learning softwarelibraries may be used to implement the trained statistical classifierincluding, but not limited to, Theano, Torch, Caffe, Keras, andTensorFlow. These libraries may be used for training a statisticalclassifier (e.g., a neural network) and/or using a trained statisticalclassifier. The trained statistical classifier may be trained using anysuitable training technique including any of the neural network trainingtechniques (e.g., gradient descent) described above. It should also beappreciated that the trained statistical classifier is not limited tobeing a neural network and may be any other suitable type of statisticalclassifier (e.g., a support vector machine, a graphical model, aBayesian classifier, a decision tree classifier, etc.), as aspects ofthe technology described herein are not limited in this respect.

In some embodiments, the trained statistical classifier may be one ofthe neural networks described above with reference to FIGS. 9A-9C orFIG. 10. Such a trained statistical classifier may identify pointlocations in MRI image data. For example, such a trained statisticalclassifier may be used to identify locations of endpoints of first andsecond orthogonal diameters of a hemorrhage. As another example, such atrained statistical classifier may be used to identify locations ofcorners of a bounding box of a hemorrhage.

In other embodiments, the trained statistical classifier may be a fullyconvolutional neural network having an architecture as illustrated inFIG. 14. Such a trained statistical classifier may be used to identifythe boundary of the hemorrhage. Training such a neural network mayinvolve zero-padding training images, using convolutional kernels ofsize 3 and stride 1, using a max pooling kernel with of size 2, anddeconvolution (upscale and convolution) kernels with size 6 and size 2.The output of the neural network may identify the boundary of thehemorrhage.

In yet other embodiments, the trained statistical classifier may be aconvolutional neural network having an architecture as illustrated inFIG. 15. Such a trained statistical classifier may be used to identifythe boundary of the hemorrhage by classifying individual voxels, whichapproach has the advantage of higher invariance to the location of thelesion. The neural network uses convolutional kernels with size 5 andstride 1 at the first layer and kernels with size 3 in the subsequentlayers. This building block can be repeated for different sizes of theinput neighborhood (25 as shown, 20, 15, or larger, 30, 35). Largerneighborhoods use larger initial kernel size (e.g., 7). The feature mapsare merged in the last feature layer and combined to yield a singleprediction.

It should be appreciated that the neural network architecturesillustrated in FIGS. 14 and 15 are illustrative and that variations ofthese architectures are possible. For example, one or more other neuralnetwork layers (e.g., a convolutional layer, a deconvolutional layer, arectified linear unit layer, an upsampling layer, a concatenate layer, apad layer, etc.) may be introduced to any of the neural networkarchitectures of FIGS. 14 and 15 as an additional one or more layersand/or instead of one or more layers part of the illustratedarchitectures. As another example, the dimensionality of one or morelayers may be varied and/or the kernel size for one or moreconvolutional, pooling, and/or deconvolutional layers may be varied.

In some embodiments, when multiple slices are being processed by theneural network, the convolutions may be two-dimensional (2D) orthree-dimensional (3D) convolutions. In some embodiments, the processingmay be slice based so that features are calculated for each slice usinginformation from the slice and one or more of its neighboring slices(the slice itself or the slice itself and one or more of its neighboringslices). In other embodiments, the processing may be a fully-3Dprocessing pipeline such that features for multiple slices are computedconcurrently using data present in all of the slices.

Next, process 1300 proceeds to act 1308, where the next MR image data isacquired. The next MR image data is acquired after the initial MR dataacquired. Thus, although, in some embodiments, acts 1304 and 1306 may beperformed after act 1308 is performed, act 1308 is generally performedafter act 1302. The next MR image data may be acquired immediatelyfollowing acquisition of the initial MR image data, or may be obtainedafter a desired period of delay (e.g., within 1, 2, 3, 4, 5, 10, 15, 20minutes, within one hour, within two hours, etc.). As with the initialMR image data, the next MR image data may be of any form (e.g., a 3Dvolumetric image, a 2D image, k-space MR data, etc.). In someembodiments, the initial MR data and the next MR image data are of thesame type. For example, each of the initial and next MR data may includeone or more two-dimensional images of one or more respective (e.g.,neighboring) brain slices. For example, the initial MR data may includemultiple images of neighboring slices obtained at a first time and thenext MR data may include multiple images of the same neighboring slicesobtained at a second time later than the first time.

Next, process 1300 proceeds to act 1310 where the next MR image data isprovided as input to the trained statistical classifier to obtain thecorresponding next output. In some embodiments, prior to being providedto the trained statistical classifier, the next MR image data may bepre-processed, for example, by resampling, interpolation, affinetransformation, and/or using any other suitable pre-processingtechniques, as aspects of the technology described herein are notlimited in this respect. The next MR image data may be preprocessed inthe same way as the initial MR data was preprocessed.

In some embodiments, the output of the trained statistical classifiermay be used to identify, at act 1312, updated value(s) of feature(s)indicative of the size of a hemorrhage in the patient's brain. In someembodiments, the features may be a first maximum diameter of thehemorrhage in a first direction and a maximum diameter of the hemorrhagein a second direction, which is orthogonal to the first direction. Theupdated values may indicate the updated lengths of the diameters and/orthe endpoints of the diameters (from which the lengths may be derived).In some embodiments, the features may be corners of a bounding boxbounding the perimeter of the hemorrhage and the updated values may bethe updated locations of the corners. In some embodiments, the featuresmay specify the boundary of the hemorrhage and the updated values may bethe updated locations of one or more points along the segmentedboundary. The output of the trained statistical classifier may specifythe updated value(s) directly or indirectly. In the latter case, thevalue(s) may be derived from information included in the output of thetrained statistical classifier.

In some embodiments, updated value(s) of the feature(s) obtained at act1306 may be used to obtain an updated estimate of the size of thehemorrhage. For example, when the updated values may be used todetermine updated lengths of maximum orthogonal diameters of thehemorrhage, the updated lengths may be used to estimate the volume ofthe hemorrhage (e.g., according to above-described ABC/2 method). Asanother example, when the updated values specify the boundary of ahemorrhage, the boundary information may be used to estimate the updatedarea of the hemorrhage in the slice.

Next, process 1300 proceeds to act 1314, where it is determined whetherthe size of the hemorrhage has changed and, if so, by how much. Thedetermination may be made using the initial and updated value(s)obtained at acts 1306 and 1312, respectively. For example, in someembodiments, the initial value(s) obtained at act 1306 may be used toobtain an initial estimate of size (e.g., volume, area, etc.) for thehemorrhage and the updated value(s) obtained at act 1312 may be used toobtained an updated estimate of the size. In turn, the initial andupdated size estimates may be used to determine whether the size of thehemorrhage changed (e.g., by evaluating their difference) and, if so, byhow much.

Next, process 1300 proceeds to decision block 1316, where it isdetermined whether to continue monitoring the size of the hemorrhage forany changes. This determination may be performed in any suitable way(e.g., by determining whether a threshold number of iterations have beenperformed, based on a schedule, based on manual input provided by aclinician, etc.), as aspects of the technology described herein are notlimited in this respect. When it is determined that monitoring is tocontinue, process 1300 returns to block 1308 and acts 1308-1314 arerepeated again (with newly obtained MR data being compared to the mostrecently previously obtained MR data). On the other hand, when it isdetermined that monitoring need not continue, process 1300 completes.

FIG. 16 is a diagram of an illustrative computer system on whichembodiments described herein may be implemented. An illustrativeimplementation of a computer system 1600 that may be used in connectionwith any of the embodiments of the disclosure provided herein is shownin FIG. 16. For example, the processes described with reference to FIGS.8 and 13 may be implemented on and/or using computer system 1600. Thecomputer system 1600 may include one or more processors 1610 and one ormore articles of manufacture that comprise non-transitorycomputer-readable storage media (e.g., memory 1620 and one or morenon-volatile storage media 1630). The processor 1610 may control writingdata to and reading data from the memory 1620 and the non-volatilestorage device 1630 in any suitable manner, as the aspects of thedisclosure provided herein are not limited in this respect. To performany of the functionality described herein, the processor 1610 mayexecute one or more processor-executable instructions stored in one ormore non-transitory computer-readable storage media (e.g., the memory1620), which may serve as non-transitory computer-readable storage mediastoring processor-executable instructions for execution by the processor1610.

Having thus described several aspects and embodiments of the technologyset forth in the disclosure, it is to be appreciated that variousalterations, modifications, and improvements will readily occur to thoseskilled in the art. Such alterations, modifications, and improvementsare intended to be within the spirit and scope of the technologydescribed herein. For example, those of ordinary skill in the art willreadily envision a variety of other means and/or structures forperforming the function and/or obtaining the results and/or one or moreof the advantages described herein, and each of such variations and/ormodifications is deemed to be within the scope of the embodimentsdescribed herein. Those skilled in the art will recognize, or be able toascertain using no more than routine experimentation, many equivalentsto the specific embodiments described herein. It is, therefore, to beunderstood that the foregoing embodiments are presented by way ofexample only and that, within the scope of the appended claims andequivalents thereto, inventive embodiments may be practiced otherwisethan as specifically described. In addition, any combination of two ormore features, systems, articles, materials, kits, and/or methodsdescribed herein, if such features, systems, articles, materials, kits,and/or methods are not mutually inconsistent, is included within thescope of the present disclosure.

The above-described embodiments can be implemented in any of numerousways. One or more aspects and embodiments of the present disclosureinvolving the performance of processes or methods may utilize programinstructions executable by a device (e.g., a computer, a processor, orother device) to perform, or control performance of, the processes ormethods. In this respect, various inventive concepts may be embodied asa computer readable storage medium (or multiple computer readablestorage media) (e.g., a computer memory, one or more floppy discs,compact discs, optical discs, magnetic tapes, flash memories, circuitconfigurations in Field Programmable Gate Arrays or other semiconductordevices, or other tangible computer storage medium) encoded with one ormore programs that, when executed on one or more computers or otherprocessors, perform methods that implement one or more of the variousembodiments described above. The computer readable medium or media canbe transportable, such that the program or programs stored thereon canbe loaded onto one or more different computers or other processors toimplement various ones of the aspects described above. In someembodiments, computer readable media may be non-transitory media.

The terms “program” or “software” are used herein in a generic sense torefer to any type of computer code or set of computer-executableinstructions that can be employed to program a computer or otherprocessor to implement various aspects as described above. Additionally,it should be appreciated that according to one aspect, one or morecomputer programs that when executed perform methods of the presentdisclosure need not reside on a single computer or processor, but may bedistributed in a modular fashion among a number of different computersor processors to implement various aspects of the present disclosure.

Computer-executable instructions may be in many forms, such as programmodules, executed by one or more computers or other devices. Generally,program modules include routines, programs, objects, components, datastructures, etc. that perform particular tasks or implement particularabstract data types. Typically the functionality of the program modulesmay be combined or distributed as desired in various embodiments.

Also, data structures may be stored in computer-readable media in anysuitable form. For simplicity of illustration, data structures may beshown to have fields that are related through location in the datastructure. Such relationships may likewise be achieved by assigningstorage for the fields with locations in a computer-readable medium thatconvey relationship between the fields. However, any suitable mechanismmay be used to establish a relationship between information in fields ofa data structure, including through the use of pointers, tags or othermechanisms that establish relationship between data elements.

When implemented in software, the software code can be executed on anysuitable processor or collection of processors, whether provided in asingle computer or distributed among multiple computers.

Further, it should be appreciated that a computer may be embodied in anyof a number of forms, such as a rack-mounted computer, a desktopcomputer, a laptop computer, or a tablet computer, as non-limitingexamples. Additionally, a computer may be embedded in a device notgenerally regarded as a computer but with suitable processingcapabilities, including a Personal Digital Assistant (PDA), a smartphoneor any other suitable portable or fixed electronic device.

Also, a computer may have one or more input and output devices. Thesedevices can be used, among other things, to present a user interface.Examples of output devices that can be used to provide a user interfaceinclude printers or display screens for visual presentation of outputand speakers or other sound generating devices for audible presentationof output. Examples of input devices that can be used for a userinterface include keyboards, and pointing devices, such as mice, touchpads, and digitizing tablets. As another example, a computer may receiveinput information through speech recognition or in other audibleformats.

Such computers may be interconnected by one or more networks in anysuitable form, including a local area network or a wide area network,such as an enterprise network, and intelligent network (IN) or theInternet. Such networks may be based on any suitable technology and mayoperate according to any suitable protocol and may include wirelessnetworks, wired networks or fiber optic networks.

Also, as described, some aspects may be embodied as one or more methods.The acts performed as part of the method may be ordered in any suitableway. Accordingly, embodiments may be constructed in which acts areperformed in an order different than illustrated, which may includeperforming some acts simultaneously, even though shown as sequentialacts in illustrative embodiments.

All definitions, as defined and used herein, should be understood tocontrol over dictionary definitions, definitions in documentsincorporated by reference, and/or ordinary meanings of the definedterms.

The indefinite articles “a” and “an,” as used herein in thespecification and in the claims, unless clearly indicated to thecontrary, should be understood to mean “at least one.”

The phrase “and/or,” as used herein in the specification and in theclaims, should be understood to mean “either or both” of the elements soconjoined, i.e., elements that are conjunctively present in some casesand disjunctively present in other cases. Multiple elements listed with“and/or” should be construed in the same fashion, i.e., “one or more” ofthe elements so conjoined. Other elements may optionally be presentother than the elements specifically identified by the “and/or” clause,whether related or unrelated to those elements specifically identified.Thus, as a non-limiting example, a reference to “A and/or B”, when usedin conjunction with open-ended language such as “comprising” can refer,in one embodiment, to A only (optionally including elements other thanB); in another embodiment, to B only (optionally including elementsother than A); in yet another embodiment, to both A and B (optionallyincluding other elements); etc.

As used herein in the specification and in the claims, the phrase “atleast one,” in reference to a list of one or more elements, should beunderstood to mean at least one element selected from any one or more ofthe elements in the list of elements, but not necessarily including atleast one of each and every element specifically listed within the listof elements and not excluding any combinations of elements in the listof elements. This definition also allows that elements may optionally bepresent other than the elements specifically identified within the listof elements to which the phrase “at least one” refers, whether relatedor unrelated to those elements specifically identified. Thus, as anon-limiting example, “at least one of A and B” (or, equivalently, “atleast one of A or B,” or, equivalently “at least one of A and/or B”) canrefer, in one embodiment, to at least one, optionally including morethan one, A, with no B present (and optionally including elements otherthan B); in another embodiment, to at least one, optionally includingmore than one, B, with no A present (and optionally including elementsother than A); in yet another embodiment, to at least one, optionallyincluding more than one, A, and at least one, optionally including morethan one, B (and optionally including other elements); etc.

Also, the phraseology and terminology used herein is for the purpose ofdescription and should not be regarded as limiting. The use of“including,” “comprising,” or “having,” “containing,” “involving,” andvariations thereof herein, is meant to encompass the items listedthereafter and equivalents thereof as well as additional items.

In the claims, as well as in the specification above, all transitionalphrases such as “comprising,” “including,” “carrying,” “having,”“containing,” “involving,” “holding,” “composed of,” and the like are tobe understood to be open-ended, i.e., to mean including but not limitedto. Only the transitional phrases “consisting of” and “consistingessentially of” shall be closed or semi-closed transitional phrases,respectively.

What is claimed is:
 1. A method of determining change in size of anabnormality in a brain of a patient positioned within a low-fieldmagnetic resonance imaging (MRI) device, the method comprising: whilethe patient remains positioned within the low-field MRI device:acquiring first magnetic resonance (MR) image data of a patient's brain;providing the first MR image data as input to a trained statisticalclassifier to obtain corresponding first output; acquiring second MRimage data of the patient's brain subsequent to acquiring the first MRimage data; providing the second MR image data as input to the trainedstatistical classifier to obtain corresponding second output;determining the change in the size of the abnormality in the patient'sbrain using the first output and the second output.
 2. The method ofclaim 1, further comprising: while the patient remains positioned withinthe low-field MRI device: identifying, using the first output, at leastone initial value of at least one feature indicative of a size of anabnormality in the patient's brain; identifying, using the secondoutput, at least one updated value of the at least one featureindicative of the size of the abnormality in the patient's brain,wherein determining the change in the size of the abnormality isperformed using the at least one initial value of the at least onefeature and the at least one updated value of the at least one feature.3. The method of claim 2, wherein identifying, using the first output,the at least one initial value of the at least one feature indicative ofthe size of the abnormality in the patient's brain comprises:identifying one or more first values indicative of a first diameter ofthe abnormality.
 4. The method of claim 3, wherein the identifyingfurther comprises identifying one or more second values indicative of asecond diameter of the abnormality orthogonal to the first diameter. 5.The method of claim 2, wherein determining the change in the size of theabnormality using the at least one initial value of the at least onefeature and the at least one updated value of the at least one featurecomprises: determining an initial size of the abnormality using the atleast one initial value of the at least one feature; determining anupdated size of the abnormality using the at least one updated value ofthe at least one feature; and determining the change in the size of theabnormality using the determined initial and updated sizes of theabnormality.
 6. The method of claim 1, wherein the trained statisticalclassifier comprises a multi-layer neural network.
 7. The method ofclaim 1, wherein the trained statistical classifier comprises aconvolutional neural network.
 8. The method of claim 1, wherein thetrained statistical classifier comprises a fully convolutional neuralnetwork.
 9. The method of claim 1, wherein the second MR image data isobtained within one hour of the first MR image data.
 10. The method ofclaim 1, further comprising repeating acquiring MR image data to obtaina sequence of frames of MR image data.
 11. The method of claim 10,wherein the sequence of frames is acquired over a period of time greaterthan an hour while the patient remains positioned within the low-fieldmagnetic resonance imaging device.
 12. The method of claim 10, whereinthe sequence of frames is acquired over a period of time greater thantwo hours while the patient remains positioned within the low-fieldmagnetic resonance imaging device.
 13. The method of claim 10, whereinthe sequence of frames is acquired over a period of time greater thanfive hours while the patient remains positioned within the low-fieldmagnetic resonance imaging device.
 14. The method of claim 1, whereinthe abnormality comprises a hemorrhage.
 15. The method of claim 1,wherein the abnormality comprises a hemorrhage, a lesion, an edema, astroke core, a stroke penumbra, and/or swelling.
 16. A low-fieldmagnetic resonance imaging (MRI) device configured to determine changein size of an abnormality in a brain of a patient, the low-field MRIdevice comprising: a plurality of magnetic components, including: a B0magnet configured to produce, at least in part, a B0 magnetic field; atleast one gradient magnet configured to spatially encode magneticresonance data; and at least one radio frequency coil configured tostimulate a magnetic resonance response and detect magnetic componentsconfigured to, when operated, acquire magnetic resonance image data; andat least one controller configured to operate the plurality of magnetcomponents to, while the patient remains positioned within the low-fieldmagnetic resonance imaging device, acquire first magnetic resonance (MR)image data of the patient's brain, and acquire second MR image data ofthe patient's brain subsequent to acquiring the first MR image data,wherein the at least one controller further configured to perform:providing the first and second MR image data as input to a trainedstatistical classifier to obtain corresponding first output and secondoutput; acquiring second MR image data of the patient's brain subsequentto acquiring the first MR image data; determining the change in the sizeof the abnormality using the first output and second output.
 17. Atleast one non-transitory computer-readable storage medium storingprocessor-executable instructions that, when executed by at least onecomputer hardware processor, cause the at least one computer hardwareprocessor, to perform method of determining change in size of anabnormality in a brain of a patient positioned within a low-fieldmagnetic resonance imaging (MRI) device, the method comprising: whilethe patient remains positioned within the low-field MRI device:acquiring first magnetic resonance (MR) image data of the patient'sbrain; providing the first MR image data as input to a trainedstatistical classifier to obtain corresponding first output; acquiringsecond MR image data of the patient's brain subsequent to acquiring thefirst MR image data; providing the second MR image data as input to thetrained statistical classifier to obtain corresponding second output;determining the change in the size of the abnormality using the firstoutput and the second output.
 18. A system, comprising: at least onecomputer hardware processor; at least one non-transitorycomputer-readable storage medium storing processor-executableinstructions that, when executed by the at least one computer hardwareprocessor, cause the at least one computer hardware processor, toperform method of determining change in size of an abnormality in abrain of a patient positioned within a low-field magnetic resonanceimaging (MRI) device, the method comprising: while the patient remainspositioned within the low-field MRI device: acquiring first magneticresonance (MR) image data of the patient's brain; providing the first MRimage data as input to a trained statistical classifier to obtaincorresponding first output; acquiring second MR image data of thepatient's brain subsequent to acquiring the first MR image data;providing the second MR image data as input to the trained statisticalclassifier to obtain corresponding second output; and determining thechange in the size of the abnormality using the first output and thesecond output.
 19. The low-field magnetic resonance imaging (MRI) deviceof claim 16, the at least one controller further configured to perform:while the patient remains positioned within the low-field MRI device:identifying, using the first output, at least one initial value of atleast one feature indicative of a size of an abnormality in thepatient's brain; identifying, using the second output, at least oneupdated value of the at least one feature indicative of the size of theabnormality in the patient's brain, wherein determining the change inthe size of the abnormality is performed using the at least one initialvalue of the at least one feature and the at least one updated value ofthe at least one feature.
 20. The at least one non-transitorycomputer-readable storage medium of claim 17, the method furthercomprising: while the patient remains positioned within the low-fieldMRI device: identifying, using the first output, at least one initialvalue of at least one feature indicative of a size of an abnormality inthe patient's brain; identifying, using the second output, at least oneupdated value of the at least one feature indicative of the size of theabnormality in the patient's brain, wherein determining the change inthe size of the abnormality is performed using the at least one initialvalue of the at least one feature and the at least one updated value ofthe at least one feature.
 21. The system of claim 18, the method furthercomprising: while the patient remains positioned within the low-fieldMRI device: identifying, using the first output, at least one initialvalue of at least one feature indicative of a size of an abnormality inthe patient's brain; identifying, using the second output, at least oneupdated value of the at least one feature indicative of the size of theabnormality in the patient's brain, wherein determining the change inthe size of the abnormality is performed using the at least one initialvalue of the at least one feature and the at least one updated value ofthe at least one feature.
 22. The method of claim 2, whereinidentifying, using the first output, the at least one initial value ofthe at least one feature indicative of the size of the abnormality inthe patient's brain comprises: identifying a region in the MR image dataincluding the abnormality.